Investments - Unrealized Capital Losses 2 (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Unrealized Capital Losses	$ 603.9		$ 1,136.5
Fair value decline below amortized cost less than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	15,832.5		21,802.2
Total Unrealized Capital Losses	587.6		1,087.3
Fair value decline below amortized cost less than 20% | Residential mortgage-backed securities agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	1,605.8	[1]	2,008.9	[1]
Total Unrealized Capital Losses	39.3	[1]	57.9	[1]
Fair value decline below amortized cost less than 20% | Other ABS (Non-RMBS)
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	200.6	[1]	225.2	[1]
Total Unrealized Capital Losses	2.2	[1]	2.5	[1]
Fair value decline below amortized cost less than 20% | Total RMBS and Other ABS
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	2,306.7	[1]	2,902.6	[1]
Total Unrealized Capital Losses	75.6	[1]	95.8	[1]
Fair value decline below amortized cost greater than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	59.9		203.9
Total Unrealized Capital Losses	16.3		49.2
Fair value decline below amortized cost greater than 20% | Residential mortgage-backed securities agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	4.5	[1]	11.3	[1]
Total Unrealized Capital Losses	2.6	[1]	4.2	[1]
Fair value decline below amortized cost greater than 20% | Other ABS (Non-RMBS)
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	2.1	[1]	2.1	[1]
Total Unrealized Capital Losses	0.5	[1]	0.5	[1]
Fair value decline below amortized cost greater than 20% | Total RMBS and Other ABS
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	27.9	[1]	87.8	[1]
Total Unrealized Capital Losses	8.6	[1]	22.6	[1]
Fixed Rate | Fair value decline below amortized cost less than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	1,317.3		1,723.7
Total Unrealized Capital Losses	36.3		50.5
Fixed Rate | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	4.2		4.4
Total Unrealized Capital Losses	1.4		1.6
Floating Rate | Fair value decline below amortized cost less than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	989.4		1,178.9
Total Unrealized Capital Losses	39.3		45.3
Floating Rate | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	23.7		83.4
Total Unrealized Capital Losses	7.2		21.0
Greater than 10%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Credit Enhancement Percentage, minimum	10.00%		10.00%
Greater than 10% | Fair value decline below amortized cost less than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	387.3	[1]	407.1	[1]
Total Unrealized Capital Losses	27.8	[1]	27.6	[1]
Greater than 10% | Fair value decline below amortized cost greater than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	17.9	[1]	47.7	[1]
Total Unrealized Capital Losses	4.4	[1]	11.1	[1]
5% - 10%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Credit Enhancement Percentage, maximum	10.00%		10.00%
Credit Enhancement Percentage, minimum	5.00%		5.00%
5% - 10% | Fair value decline below amortized cost less than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	20.6	[1]	43.9	[1]
Total Unrealized Capital Losses	0.7	[1]	1.2	[1]
5% - 10% | Fair value decline below amortized cost greater than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	0	[1]	0.8	[1]
Total Unrealized Capital Losses	0	[1]	0.2	[1]
0% - 5%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Credit Enhancement Percentage, maximum	5.00%		5.00%
Credit Enhancement Percentage, minimum	0.00%		0.00%
0% - 5% | Fair value decline below amortized cost less than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	34.1	[1]	90.4	[1]
Total Unrealized Capital Losses	1.0	[1]	1.9	[1]
0% - 5% | Fair value decline below amortized cost greater than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	0	[1]	3.9	[1]
Total Unrealized Capital Losses	0	[1]	0.8	[1]
0%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Credit Enhancement Percentage, maximum	0.00%		0.00%
0% | Fair value decline below amortized cost less than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	58.3	[1]	127.1	[1]
Total Unrealized Capital Losses	4.6	[1]	4.7	[1]
0% | Fair value decline below amortized cost greater than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	3.4	[1]	22.0	[1]
Total Unrealized Capital Losses	1.1	[1]	5.8	[1]
Greater than 100%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Loan to Value Ratio, minimum	100.00%		100.00%
Greater than 100% | Fair value decline below amortized cost less than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	38.9		75.7
Total Unrealized Capital Losses	4.2		2.9
Greater than 100% | Fair value decline below amortized cost greater than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	2.6		36.4
Total Unrealized Capital Losses	0.6		8.3
90% - 100%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Loan to Value Ratio, minimum	90.00%		90.00%
Loan to Value Ratio, maximum	100.00%		100.00%
90% - 100% | Fair value decline below amortized cost less than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	80.5		156.8
Total Unrealized Capital Losses	5.1		8.6
90% - 100% | Fair value decline below amortized cost greater than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	7.1		24.1
Total Unrealized Capital Losses	1.6		5.7
80% - 90%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Loan to Value Ratio, minimum	80.00%		80.00%
Loan to Value Ratio, maximum	90.00%		90.00%
80% - 90% | Fair value decline below amortized cost less than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	99.9		151.3
Total Unrealized Capital Losses	9.1		8.4
80% - 90% | Fair value decline below amortized cost greater than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	4.8		5.9
Total Unrealized Capital Losses	1.2		1.7
Less than 80%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Loan to Value Ratio, maximum	80.00%		80.00%
Less than 80% | Fair value decline below amortized cost less than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	281.0		284.7
Total Unrealized Capital Losses	15.7		15.5
Less than 80% | Fair value decline below amortized cost greater than 20% | Residential mortgage-backed securities non-agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	6.8		8.0
Total Unrealized Capital Losses	$ 2.1		$ 2.2

[1] For purposes of this table, subprime mortgages are included in Non-agency RMBS categories.