Policyholder Account Balance, Guaranteed Minimum Crediting (Detail) - USD ($)
$ in Millions

Dec. 31, 2023

Dec. 31, 2022

Dec. 31, 2021

Dec. 31, 2020

Operating Segments

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

$ 5,514.2

$ 5,598.6

$ 5,679.6

$ 5,678.5

Colonial Life

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

852.9

852.4

849.2

843.8

Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

578.6

586.8

598.7

616.7

Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

4,082.7

4,159.4

4,231.7

$ 4,218.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0300 to 0399 | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

$ 91.9

$ 94.4

$ 98.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0300 to 0399 | Minimum | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

3.00%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0300 to 0399 | Maximum | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

3.99%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0400 and 0499 | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

$ 454.9

$ 458.4

$ 465.8

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0400 and 0499 | Minimum | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

4.00%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0400 and 0499 | Maximum | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

4.99%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0500 and 0600 | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

$ 31.8

$ 34.0

$ 34.9

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0500 and 0600 | Minimum | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

5.00%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0500 and 0600 | Maximum | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

6.00%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0400 and 0500 | Colonial Life

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

$ 852.9

$ 852.4

$ 849.2

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0400 and 0500 | Minimum | Colonial Life

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

4.00%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0400 and 0500 | Maximum | Colonial Life

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

5.00%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0300 and 0599 | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

$ 1,639.3

$ 1,697.2

$ 1,852.7

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0300 and 0599 | Minimum | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

3.00%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0300 and 0599 | Maximum | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

5.99%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0600 and 0899 | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

$ 32.3

$ 31.4

$ 29.6

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0600 and 0899 | Minimum | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

6.00%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0600 and 0899 | Maximum | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

8.99%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0900 and 1199 | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

$ 2,354.9

$ 2,378.2

$ 2,300.4

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0900 and 1199 | Minimum | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

9.00%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0900 and 1199 | Maximum | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

11.99%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 1200 and 1500 | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

$ 56.2

$ 52.6

$ 49.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 1200 and 1500 | Minimum | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

12.00%

Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 1200 and 1500 | Maximum | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, Guaranteed Minimum Credit Rating

15.00%

Policyholder Account Balance, at Guaranteed Minimum Crediting Rate | Operating Segments

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

$ 2,044.0

$ 5,373.2

$ 5,460.2

Policyholder Account Balance, at Guaranteed Minimum Crediting Rate | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

350.8

402.9

421.7

Policyholder Account Balance, at Guaranteed Minimum Crediting Rate | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

846.5

4,123.9

4,195.1

Policyholder Account Balance, at Guaranteed Minimum Crediting Rate | Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0300 to 0399 | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

91.9

94.4

98.0

Policyholder Account Balance, at Guaranteed Minimum Crediting Rate | Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0400 and 0499 | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

227.1

274.5

288.8

Policyholder Account Balance, at Guaranteed Minimum Crediting Rate | Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0500 and 0600 | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

31.8

34.0

34.9

Policyholder Account Balance, at Guaranteed Minimum Crediting Rate | Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0400 and 0500 | Colonial Life

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

846.7

846.4

843.4

Policyholder Account Balance, at Guaranteed Minimum Crediting Rate | Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0300 and 0599 | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

526.8

1,661.7

1,816.1

Policyholder Account Balance, at Guaranteed Minimum Crediting Rate | Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0600 and 0899 | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

1.3

31.4

29.6

Policyholder Account Balance, at Guaranteed Minimum Crediting Rate | Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0900 and 1199 | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

318.4

2,378.2

2,300.4

Policyholder Account Balance, at Guaranteed Minimum Crediting Rate | Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 1200 and 1500 | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

52.6

49.0

Policyholder Account Balance, above Guaranteed Minimum Crediting Rate, Range from 0001 to 0050 | Operating Segments

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

3,401.8

219.0

213.0

Policyholder Account Balance, above Guaranteed Minimum Crediting Rate, Range from 0001 to 0050 | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

190.1

183.9

177.0

Policyholder Account Balance, above Guaranteed Minimum Crediting Rate, Range from 0001 to 0050 | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

3,205.5

29.1

30.2

Policyholder Account Balance, above Guaranteed Minimum Crediting Rate, Range from 0001 to 0050 | Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0300 to 0399 | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

190.1

183.9

177.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

6.2

6.0

5.8

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

1,081.8

29.1

30.2

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

31.0

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

2,036.5

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

56.2

0.0

Policyholder Account Balance, above Guaranteed Minimum Crediting Rate, Range from 0051 to 0150 | Operating Segments

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

68.4

6.4

Policyholder Account Balance, above Guaranteed Minimum Crediting Rate, Range from 0051 to 0150 | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

37.7

0.0

Policyholder Account Balance, above Guaranteed Minimum Crediting Rate, Range from 0051 to 0150 | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

30.7

6.4

Policyholder Account Balance, above Guaranteed Minimum Crediting Rate, Range from 0051 to 0150 | Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0300 to 0399 | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

37.7

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

30.7

6.4

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, above Guaranteed Minimum Crediting Rate, Range from 0151 and Greater | Operating Segments

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, above Guaranteed Minimum Crediting Rate, Range from 0151 and Greater | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, above Guaranteed Minimum Crediting Rate, Range from 0151 and Greater | Other Insurance Product Line | Closed Block

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, above Guaranteed Minimum Crediting Rate, Range from 0151 and Greater | Policyholder Account Balance, Guaranteed Minimum Crediting Rate, Range from 0300 to 0399 | Voluntary Benefits | Unum US

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

0.0

Policyholder Account Balance, Guaranteed Minimum Crediting Rate [Line Items]

Policyholder Account Balance, before Reserves in excess of Account Balance

$ 0.0