Deferred Policy Acquisition Costs and Value of Business Acquired (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance			$ 476.4	$ 520.4
Deferrals of commissions and expenses			55.7	56.5
Amortization:
Amortization, excluding unlocking			(56.9)	(55.5)
Unlocking (1)			(59.8)	(16.3)
Interest accrued			27.9	31.1
Net amortization included in the Condensed Consolidated Statements of Operations			(88.8)	(40.7)
Change due to unrealized capital gains/losses on available-for-sale securities			(67.8)	(185.9)
Ending balance	$ 375.5	$ 350.3	375.5	350.3
Movement Analysis Of Value of Business Acquired VOBA [Roll Forward]
Beginning balance			537.4	708.7
Deferrals of commissions and expenses			3.6	4.1
Amortization, excluding unlocking			(68.1)	(53.8)
Unlocking (1)			(96.7)	(61.4)
Interest accrued			33.7	39.6
Net amortization included in the Condensed Consolidated Statements of Operations			(131.1)	(75.6)
Change due to unrealized capital gains/losses on available-for-sale securities			(67.2)	(262.3)
Ending balance	342.7	374.9	342.7	374.9
Movement Analysis of Deferred Policy Acquisition Costs and Value of Business Acquired (VOBA) [Roll Forward]
Beginning balance			1,013.8	1,229.1
Deferrals of commissions and expenses			59.3	60.6
Amortization, excluding unlocking			(125.0)	(109.3)
Unlocking (1)			(156.5)	(77.7)
Interest accrued			61.6	70.7
Net amortization included in the Condensed Consolidated Statements of Operations	(84.4)	(119.5)	(219.9)	(116.3)
Change due to unrealized capital gains/losses on available-for-sale securities			(135.0)	(448.2)
Ending balance	$ 718.2	$ 725.2	$ 718.2	$ 725.2
Minimum
Movement Analysis of Deferred Policy Acquisition Costs and Value of Business Acquired (VOBA) [Roll Forward]
Value of Business Acquired (VOBA), Interest accrued percentage			5.50%
Maximum
Movement Analysis of Deferred Policy Acquisition Costs and Value of Business Acquired (VOBA) [Roll Forward]
Value of Business Acquired (VOBA), Interest accrued percentage			7.00%
Guaranteed minimum interest rates
Amortization:
Unlocking (1)			$ (79.6)
Movement Analysis Of Value of Business Acquired VOBA [Roll Forward]
Unlocking (1)			$ (140.2)