Investments - Unrealized Capital Losses 2 (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Unrealized Capital Losses	$ 349.1		$ 1,136.5
Fair value decline below amortized cost less than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	12,489.8		21,802.2
Total Unrealized Capital Losses	344.5		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	915.5	[1]	2,008.9	[1]
Total Unrealized Capital Losses	18.5	[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	252.8	[1]	225.2	[1]
Total Unrealized Capital Losses	1.7	[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	1,611.2		2,902.6
Total Unrealized Capital Losses	44.6		95.8
Fair value decline below amortized cost greater than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	14.2		203.9
Total Unrealized Capital Losses	4.6		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	3.8	[1]	11.3	[1]
Total Unrealized Capital Losses	2.0	[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	2.0	[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	13.2		87.8
Total Unrealized Capital Losses	4.4		22.6
Fixed Rate | Fair value decline below amortized cost less than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	1,053.7		1,723.7
Total Unrealized Capital Losses	20.1		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	2.4		4.4
Total Unrealized Capital Losses	0.7		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	557.5		1,178.9
Total Unrealized Capital Losses	24.5		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	10.8		83.4
Total Unrealized Capital Losses	3.7		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	343.7	[1]	407.1	[1]
Total Unrealized Capital Losses	20.0	[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	7.4	[1]	47.7	[1]
Total Unrealized Capital Losses	1.9	[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	18.5	[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	41.0	[1]	90.4	[1]
Total Unrealized Capital Losses	0.9	[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	39.7	[1]	127.1	[1]
Total Unrealized Capital Losses	2.8	[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	0	[1]	22.0	[1]
Total Unrealized Capital Losses	0	[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	5.1		75.7
Total Unrealized Capital Losses	0.3		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	0		36.4
Total Unrealized Capital Losses	0		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	36.1		156.8
Total Unrealized Capital Losses	1.9		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	0		24.1
Total Unrealized Capital Losses	0		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	108.5		151.3
Total Unrealized Capital Losses	5.9		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	0.3		5.9
Total Unrealized Capital Losses	0.1		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	293.2		284.7
Total Unrealized Capital Losses	16.3		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	7.1		8.0
Total Unrealized Capital Losses	$ 1.8		$ 2.2

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