Deferred Acquisition Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in deferred acquisition costs
Balance at beginning of year, deferred acquisition costs	$ 3,521.3	$ 3,693.5	$ 3,540.7
Costs deferred during the year	457.0	473.5	414.9
Amortization of deferred acquisition costs	(388.1)	(347.0)	(253.5)
Adjustment related to unrealized (gains) losses on available-for-sale securities and derivative instruments	(180.5)	(298.7)	184.9
Other			(193.5)
Balance at end of year, deferred acquisition costs	$ 3,409.7	$ 3,521.3	$ 3,693.5