Deferred Policy Acquisition Costs and Value of Business Acquired (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	$ 520.4	$ 396.5	$ 476.2
Deferrals of commissions and expenses	73.5	76.9	69.8
Amortization:
Amortization	(95.7)	(106.8)	(91.0)
Interest accrued	37.5	36.2	35.9
Net amortization included in the Consolidated Statements of Operations	(58.2)	(70.6)	(55.1)
Change in unrealized capital gains/losses on available-for-sale securities	(59.3)	117.6	(94.4)
Ending balance	476.4	520.4	396.5
Movement Analysis Of Value of Business Acquired VOBA [Roll Forward]
Beginning balance	708.7	526.8	696.6
Deferrals of commissions and expenses	5.3	5.8	6.9
Amortization	(159.8)	(117.9)	(113.3)
Interest accrued	51.5	55.9	59.2
Net amortization included in the Consolidated Statements of Operations	(108.3)	(62.0)	(54.1)
Change in unrealized capital gains/losses on available-for-sale securities	(68.3)	238.1	(122.6)
Ending balance	537.4	708.7	526.8
Movement Analysis of Deferred Policy Acquisition Costs and Value of Business Acquired (VOBA) [Roll Forward]
Beginning balance	1,229.1	923.3	1,172.8
Deferrals of commissions and expenses	78.8	82.7	76.7
Amortization	(255.5)	(224.7)	(204.3)
Interest accrued	89.0	92.1	95.1
Net amortization included in the Consolidated Statements of Operations	(166.5)	(132.6)	(109.2)
Change in unrealized capital gains/losses on availabe-for-sale securities	(127.6)	355.7	(217.0)
Ending balance	1,013.8	$ 1,229.1	$ 923.3
Estimated amount of VOBA amortization expense, net of interest:
2017	46.0
2018	36.9
2019	30.9
2020	27.4
2021	$ 24.1
Minimum
Deferred Policy Acquisition Cost and Value of Business Acquired [Line Items]
Interest Accrual Rate Associated with Amortization Method of Present Value of Future Insurance Profits	5.50%	5.50%	5.50%
Maximum
Deferred Policy Acquisition Cost and Value of Business Acquired [Line Items]
Interest Accrual Rate Associated with Amortization Method of Present Value of Future Insurance Profits	7.00%	7.00%	7.00%