CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 358.2	$ 270.7	$ 51.4
Other comprehensive income, before tax:
Unrealized gains (losses) for the period	424.4	(1,337.6)	942.9
Amortization of present value of future profits and deferred acquisition costs	(27.9)	157.9	(113.5)
Amount related to premium deficiencies assuming the net unrealized gains (losses) had been realized	(46.9)	495.3	(624.6)
Reclassification adjustments:
For net realized investment (gains) losses included in net income	(18.6)	29.6	(59.0)
For amortization of the present value of future profits and deferred acquisition costs related to net realized investment gains (losses) included in net income	0.7	(0.5)	1.0
Unrealized gains (losses) on investments	331.7	(655.3)	146.8
Change related to deferred compensation plan	8.6	(0.1)	(1.4)
Other comprehensive income (loss) before tax	340.3	(655.4)	145.4
Income tax (expense) benefit related to items of accumulated other comprehensive income	(120.7)	232.9	(51.9)
Other comprehensive income (loss), net of tax	219.6	(422.5)	93.5
Comprehensive income (loss)	$ 577.8	$ (151.8)	$ 144.9