Accumulated Other Comprehensive Income (Loss) - Changes in AOCI, including Reclassification Adjustments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale securities, Before-Tax Amount:
Adjustments for amounts recognized in Net realized capital gains (losses) in the Consolidated Statements of Operations	$ 70.1	$ 65.5	$ 15.0
DAC/VOBA and sales inducements	(128.0)	356.0	(217.1)
Premium deficiency reserve adjustment	(22.7)	63.3	(47.4)
Net realized gains/losses on available-for-sale securities	274.6	(696.3)	467.3
Available-for-sale securities, Income Tax:
Adjustments for amounts recognized in Net realized capital gains (losses) in the Consolidated Statements of Operations	(24.5)	(22.9)	(5.3)
DAC/VOBA and sales inducements	44.8	(124.6)	76.0
Premium deficiency reserve adjustment	7.9	(22.2)	16.6
Net realized gains/losses on available-for-sale securities	(95.8)	239.4	(165.0)
Available-for-sale securities, After-Tax Amount:
Adjustments for amounts recognized in Net realized capital gains (losses) in the Consolidated Statements of Operations	45.6	42.6	9.7
DAC/VOBA and sales inducements	(83.2)	231.4	(141.1)
Premium deficiency reserve adjustment	(14.8)	41.1	(30.8)
Net realized gains/losses on available-for-sale securities	178.8	(456.9)	302.3
Derivatives, Before-Tax Amount:
Net unrealized capital gains/losses arising during the period, Before-Tax Amount	11.2	19.7	77.0
Adjustments for amounts recognized in Net investment income in the Condensed Consolidated Statements of Operations	(19.3)	(14.0)	(7.4)
Net unrealized gains/losses on derivatives	(8.1)	5.7	69.6
Derivatives, Income Tax:
Net unrealized capital gains/losses arising during the period, Income Tax	(3.9)	(6.9)	(27.0)
Adjustments for amounts recognized in Net investment income in the Condensed Consolidated Statements of Operations	6.8	4.9	2.6
Net unrealized gains/losses on derivatives	2.9	(2.0)	(24.4)
Derivatives, After-Tax Amount:
Net unrealized capital gains/losses arising during the period, After-Tax Amount	7.3	12.8	50.0
Adjustments for amounts recognized in Net investment income in the Condensed Consolidated Statements of Operations	(12.5)	(9.1)	(4.8)
Net unrealized gains/losses on derivatives	(5.2)	3.7	45.2
OTTI:
Other-than-temporary impairments	8.7	2.8	5.1
Change in OTTI, Income Tax	(3.0)	(1.0)	(1.8)
Change in OTTI, After-Tax Amount	5.7	1.8	3.3
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	(1.4)	(2.3)	(2.2)
Net pension and other post-employment benefits liability	(1.4)	(2.3)	(2.2)
Other comprehensive income (loss), before tax	265.1	(692.9)	534.7
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	0.5	0.8	0.8
Net pension and other post-employment benefit liability	0.5	0.8	0.8
Other comprehensive income (loss)	(92.4)	238.2	(188.6)
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	(0.9)	(1.5)	(1.4)
Net pension and other post-employment benefit liability	(0.9)	(1.5)	(1.4)
Other comprehensive income (loss), after tax	172.7	(454.7)	346.1
Fixed maturities
Available-for-sale securities, Before-Tax Amount:
Net unrealized gains/losses on securities	345.9	(1,183.7)	713.0
Available-for-sale securities, Income Tax:
Net unrealized gains/losses on securities	(120.8)	410.0	(251.0)
Available-for-sale securities, After-Tax Amount:
Net unrealized gains/losses on securities	225.1	(773.7)	462.0
Equity securities
Available-for-sale securities, Before-Tax Amount:
Net unrealized gains/losses on securities	0.6	(0.2)	(1.3)
Available-for-sale securities, Income Tax:
Net unrealized gains/losses on securities	(0.2)	0.1	0.5
Available-for-sale securities, After-Tax Amount:
Net unrealized gains/losses on securities	$ 0.4	$ (0.1)	$ (0.8)