Deferred Acquisition Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in deferred acquisition costs
Balance at beginning of year, deferred acquisition costs	$ 3,380.2	$ 3,276.1	$ 2,993.0
Costs deferred during the year	421.8	402.3	390.3
Amortization of deferred acquisition costs	(234.6)	(285.1)	(270.8)
Adjustment related to unrealized (gains) losses on available-for-sale securities and derivative instruments	(26.7)	(13.1)	164.5
Other			(0.9)
Balance at end of year, deferred acquisition costs	$ 3,540.7	$ 3,380.2	$ 3,276.1