Derivative Financial Instruments - Fair Value Hedges (Details) - Fair Value Hedges - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Effect of derivatives in hedging relationships and the related hedged items on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	$ 1.2	$ 7.0	$ 1.5	$ 2.7
Amount of gain (loss) recognized in net income on related hedged item	(1.2)	(6.5)	(1.8)	(2.2)
Gain (loss) on periodic settlements on interest rate and foreign exchange contracts in fair value hedge of fixed maturities, available-for-sale reported in net investment income	(2.4)	(9.6)	(8.1)	(34.6)
Gain (loss) on periodic settlements on interest rate and foreign exchange contracts in fair value hedge of investment contracts reported in benefits, claims and settlement expenses	0.1	0.6	0.9	2.0
Interest rate contracts | Fixed maturities, available-for-sale
Effect of derivatives in hedging relationships and the related hedged items on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	1.1	8.9	2.1	1.3
Amount of gain (loss) recognized in net income on related hedged item	(1.1)	(8.5)	(2.4)	(0.9)
Interest rate contracts | Investment contracts
Effect of derivatives in hedging relationships and the related hedged items on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	0.1	(1.9)	(0.6)	1.4
Amount of gain (loss) recognized in net income on related hedged item	$ (0.1)	$ 2.0	$ 0.6	$ (1.3)