Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Net Investment Income
Investment Income, Interest and Dividend		$ 574.3	$ 579.6	$ 1,133.6	$ 1,176.3
Less Investment Expenses		7.8	7.6	15.3	16.1
Less Investment Income on PFA Assets		3.0	3.0	6.1	6.2
Net Investment Income		563.5	569.0	1,112.2	1,154.0
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend		471.4	542.5	941.2	1,082.3
Derivatives
Net Investment Income
Investment Income, Interest and Dividend		17.7	20.7	33.9	40.9
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend		24.7	28.0	51.0	57.0
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend		5.0	5.0	9.8	9.8
Perpetual Preferred Securities
Net Investment Income
Investment Income, Interest and Dividend	[1]	2.0	10.6	5.6	(6.5)
Private Equity Partnerships
Net Investment Income
Investment Income, Interest and Dividend	[2]	51.9	(31.3)	87.8	(20.9)
Other Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend		1.3	1.7	3.4	4.9
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend		$ 0.3	$ 2.4	$ 0.9	$ 8.8

[1] The net unrealized gain recognized in net investment income for the three and six months ended June 30, 2021 related to perpetual preferred securities still held at June 30, 2021 was $1.5 million and $4.2 million, respectively. The net unrealized gain (loss) recognized in net investment income for the three and six months ended June 30, 2020 related to perpetual preferred securities still held at June 30, 2020 was $10.0 million and $(7.9) million, respectively.

[2] The net unrealized gain recognized in net investment income for the three and six months ended June 30, 2021 related to private equity partnerships still held at June 30, 2021 was $32.0 million and $59.0 million, respectively. The net unrealized loss recognized in net investment income for the three and six months ended June 30, 2020 related to private equity partnerships still held at June 30, 2020 was $41.0 million and $35.6 million, respectively. See Note 3 for further discussion of private equity partnerships.