Accumulated Other Comprehensive Income (Loss) - Changes in AOCI, including Reclassification Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Available-for-sale securities, Before-Tax Amount:
Other	$ 0.0	$ (0.1)	$ 0.0	$ 0.0
Adjustments for amounts recognized in Net realized capital gains (losses) in the Condensed Consolidated Statements of Operations	1.6	6.4	20.7	47.6
DAC/VOBA and sales inducements	(92.6)	(190.5)	(125.9)	(412.3)
Premium deficiency reserve adjustment	(16.2)	(27.3)	(21.0)	(45.5)
Unrealized gains/losses on securities	221.0	449.5	317.2	958.7
Available-for-sale securities, Income Tax:
Other	0.0	0.0	0.0	0.0
Adjustments for amounts recognized in Net realized capital gains (losses) in the Condensed Consolidated Statements of Operations	(0.5)	(2.3)	(7.2)	(16.7)
DAC/VOBA and sales inducements	32.3	66.7	44.0	144.3
Premium deficiency reserve adjustment	5.7	9.5	7.4	15.9
Change in unrealized gains/losses on available-for-sale securities	(76.6)	(155.1)	(109.9)	(330.3)
Available-for-sale securities, After-Tax Amount:
Other	0.0	(0.1)	0.0	0.0
Adjustments for amounts recognized in Net realized capital gains (losses) in the Condensed Consolidated Statements of Operations	1.1	4.1	13.5	30.9
DAC/VOBA and sales inducements	(60.3)	(123.8)	(81.9)	(268.0)
Premium deficiency reserve adjustment	(10.5)	(17.8)	(13.6)	(29.6)
Change in unrealized gains/losses on available-for-sale securities	144.4	294.4	207.3	628.4
OTTI:
Other-than-temporary impairments	0.2	0.4	1.8	0.8
Change in OTTI, Income Tax	(0.1)	(0.2)	(0.6)	(0.3)
Change in OTTI, After-Tax Amount	0.1	0.2	1.2	0.5
Derivatives, Before-Tax Amount:
Change in unrealized capital gains/losses arising during the period, Before-Tax Amount	(10.0)	2.8	(19.3)	15.7
Adjustments for amounts recognized in Net investment income in the Condensed Consolidated Statements of Operations	(5.8)	(4.5)	(11.6)	(8.7)
Change in unrealized gains/losses on derivatives	(15.8)	(1.7)	(30.9)	7.0
Derivatives, Income Tax:
Change in unrealized capital gains/losses arising during the period, Income Tax	3.5	(1.0)	6.8	(5.5)
Adjustments for amounts recognized in Net investment income in the Condensed Consolidated Statements of Operations	2.1	1.5	4.1	3.0
Change in unrealized gains/losses on derivatives	5.6	0.5	10.9	(2.5)
Derivatives, After-Tax Amount:
Change in unrealized capital gains/losses arising during the period, After-Tax Amount	(6.5)	1.8	(12.5)	10.2
Adjustments for amounts recognized in Net investment income in the Condensed Consolidated Statements of Operations	(3.7)	(3.0)	(7.5)	(5.7)
Change in unrealized gains/losses on derivatives	(10.2)	(1.2)	(20.0)	4.5
Pension and other post-employment benefit liability, Before-Tax Amount:
Amortization of prior service cost recognized in Operating expenses in the Condensed Consolidated Statements of Operations	(0.5)	(0.5)	(1.1)	(1.1)
Change in pension and other post-employment benefit liability	(0.5)	(0.5)	(1.1)	(1.1)
Other comprehensive income (loss), before tax	204.7	447.3	285.2	964.6
Pension and other post-employment benefit liability, Income Tax:
Amortization of prior service cost recognized in Operating expenses in the Condensed Consolidated Statements of Operations	0.2	0.2	0.4	0.4
Change in pension and other post-employment benefit liability	0.2	0.2	0.4	0.4
Other comprehensive income (loss)	(70.8)	(154.4)	(98.6)	(332.4)
Pension and other post-employment benefit liability, After-Tax Amount:
Amortization of prior service cost recognized in Operating expenses in the Condensed Consolidated Statements of Operations	(0.3)	(0.3)	(0.7)	(0.7)
Change in pension and other post-employment benefit liability	(0.3)	(0.3)	(0.7)	(0.7)
Other comprehensive income (loss), after tax	133.9	292.9	186.6	632.2
Fixed maturities
Available-for-sale securities, Before-Tax Amount:
Change in unrealized gains/losses on securities	327.6	659.9	440.9	1,366.9
Available-for-sale securities, Income Tax:
Change in unrealized gains/losses on securities	(113.9)	(228.6)	(153.3)	(473.1)
Available-for-sale securities, After-Tax Amount:
Change in unrealized gains/losses on securities	213.7	431.3	287.6	893.8
Equity securities
Available-for-sale securities, Before-Tax Amount:
Change in unrealized gains/losses on securities	0.4	0.7	0.7	1.2
Available-for-sale securities, Income Tax:
Change in unrealized gains/losses on securities	(0.1)	(0.2)	(0.2)	(0.4)
Available-for-sale securities, After-Tax Amount:
Change in unrealized gains/losses on securities	$ 0.3	$ 0.5	$ 0.5	$ 0.8