Investment Securities (Fair value rollforward) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013		Dec. 31, 2013		Dec. 31, 2012
Investment securities
Other Investments	$ 143.0				$ 139.6
Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	558.7	[1]			469.5	[1]
Amortization/accretion	0	[1]
Total net realized and unrealized gains (losses)	8.6	[1]
Purchases	140.6	[1]
Sales	(60.0)	[1]
Transfers in	0	[1]
Transfers out	0	[1]
Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	1,693.7				1,753.1
Amortization/accretion	(2.7)
Total net realized and unrealized gains (losses)	6.9
Purchases	347.8
Sales	(411.4)
Transfers in	0
Transfers out	0
Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	213.5	[2]	201.8	[2]	201.9	[2]	198.9	[2]
Amortization/accretion	0	[2]	0	[2]
Total net realized and unrealized gains (losses)	3.4	[2]	3.8	[2]
Purchases	9.8	[2]	3.6	[2]
Sales	(1.6)	[2]	(4.5)	[2]
Transfers in	0	[2]	0	[2]
Transfers out	0	[2]	0	[2]
Tax Development Fund [Member]
Investment securities
Other Investments	19.3		20.6		19.7
Fixed maturity investments | Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	211.7				132.7
Amortization/accretion	0
Total net realized and unrealized gains (losses)	0.1
Purchases	114.6
Sales	(35.7)
Transfers in	0
Transfers out	0
Fixed maturity investments | Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	1,664.4				1,722.6
Amortization/accretion	(2.6)
Total net realized and unrealized gains (losses)	5.0
Purchases	344.0
Sales	(404.6)
Transfers in	0
Transfers out	0
Fixed maturity investments | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	89.7		78.0		81.9		76.1
Amortization/accretion	0		0
Total net realized and unrealized gains (losses)	0.3		0.1
Purchases	7.5		1.8
Sales	0		0
Transfers in	0		0
Transfers out	0		0
Common equity securities | Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	347.0				336.8
Amortization/accretion	0
Total net realized and unrealized gains (losses)	8.5
Purchases	26.0
Sales	(24.3)
Transfers in	0
Transfers out	0
Common equity securities | Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	0				0
Amortization/accretion	0
Total net realized and unrealized gains (losses)	0
Purchases	0
Sales	0
Transfers in	0
Transfers out	0
Common equity securities | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	0.1		0.1		0.1		0.1
Amortization/accretion	0		0
Total net realized and unrealized gains (losses)	0		0
Purchases	0		0
Sales	0		0
Transfers in	0		0
Transfers out	0		0
Convertible fixed maturity investments | Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	0				0
Amortization/accretion	0
Total net realized and unrealized gains (losses)	0
Purchases	0
Sales	0
Transfers in	0
Transfers out	0
Convertible fixed maturity investments | Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	29.3				30.5
Amortization/accretion	(0.1)
Total net realized and unrealized gains (losses)	1.9
Purchases	3.8
Sales	(6.8)
Transfers in	0
Transfers out	0
Convertible fixed maturity investments | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	0		0		0		0
Amortization/accretion	0		0
Total net realized and unrealized gains (losses)	0		0
Purchases	0		0
Sales	0		0
Transfers in	0		0
Transfers out	0		0
Other investments | Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	0				0
Amortization/accretion	0
Total net realized and unrealized gains (losses)	0
Purchases	0
Sales	0
Transfers in	0
Transfers out	0
Other investments | Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	0				0
Amortization/accretion	0
Total net realized and unrealized gains (losses)	0
Purchases	0
Sales	0
Transfers in	0
Transfers out	0
Other investments | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt securities issued by corporations	123.7		123.7		119.9		122.7
Amortization/accretion	0		0
Total net realized and unrealized gains (losses)	3.1		3.7
Purchases	2.3		1.8
Sales	(1.6)		(4.5)
Transfers in	0		0
Transfers out	$ 0		$ 0

[1] Excludes short-term investments which are deemed to have a Level 1 designation.

[2] Excludes the carrying value of $19.3 million and $20.6 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method as of March 31, 2014 and 2013, respectively.