Deferred Policy Acquisition Costs (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Changes in Deferred Policy Acquisition Costs:
Deferred policy acquisition costs, beginning of period	$ 470.1	$ 425.5	$ 488.2
Policy acquisition costs deferred	83.7	66.7	69.8
Costs amortized to expenses:
Recurring costs	(91.6)	(76.8)	(101.2)
Assumption unlocking	(7.7)	4.6	(1.7)
Realized investment gains (losses)	14.4	(11.2)	2.4
Offsets to net unrealized investment gains or losses included in AOCI	(38.0)	61.3	(32.0)
Balance, end of period	$ 430.9	$ 470.1	$ 425.5