Debt, Letter of Credit Facilities and Trust Arrangements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended			0 Months Ended	3 Months Ended	6 Months Ended		0 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended		0 Months Ended	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Oct. 05, 2012 2022 Senior Notes	Jun. 30, 2013 2022 Senior Notes	Jun. 30, 2013 2022 Senior Notes	Dec. 31, 2012 2022 Senior Notes	Nov. 05, 2012 2013 Senior notes	Jun. 30, 2012 2013 Senior notes	Jun. 30, 2012 2013 Senior notes	Dec. 31, 2003 2013 Senior notes	Jun. 30, 2013 2013 Senior notes	Feb. 07, 2012 Trust Preferred Securities	Jan. 31, 2006 Trust Preferred Securities	Jun. 30, 2013 Trust Preferred Securities	Jun. 30, 2012 Trust Preferred Securities	Jun. 30, 2013 Trust Preferred Securities	Jun. 30, 2012 Trust Preferred Securities	Dec. 31, 2011 Trust Preferred Securities	Dec. 31, 2012 Trust Preferred Securities	Jun. 30, 2013 Syndicated Secured Facilities	Jun. 30, 2013 Four Year Committed Facility	Jun. 30, 2013 Four Year Committed Facility Minimum	Jun. 30, 2013 Four Year Committed Facility Maximum	Jun. 30, 2013 Bilateral Facility
Debt and other financing arrangements
Increase in borrowings						$ 300.0							$ 250.0			$ 100.0
Principal amount of debt outstanding										227.9				227.9
Fixed rate of interest (as a percent)							4.70%	4.70%					6.125%				4.905%		4.905%
Percentage of par value at which debt was issued						99.682%
Effective yield to investors (as a percent)								4.74%
Proceeds from issuance of Senior Unsecured Debt, net of issuance discounts and debt issuance costs						296.4
Debt issuance costs capitalized						2.7
Carrying value of debt	399.1		399.1		399.1		299.1	299.1	299.1								100.0		100.0			100.0
Interest incurred on debt	4.7	4.8	9.4	9.8			3.5	7.0			3.5	7.0					1.1	1.1	2.1	2.2
Principal amount of debt redeemed										237.6
Redemption price, as percentage of principal amount of debt instrument										104.20%
Interest paid								7.4			7.0								2.1	2.2
Floating interest rate, base rate																					3-month LIBOR
Floating interest rate, basis points added to base rate (as a percent)																					3.80%
Floating rate, low end of range (as a percent)																			4.07%	4.26%
Floating rate, high end of range (as a percent)																			4.11%	4.38%
Swap agreement term															5 years
Letter of Credit Facilities
Cash and investments collateral for letter of credit facilities	32.1		32.1		128.0
Total Capacity																								75.0			75.0
Amount Drawn																								$ 2.3			$ 23.0
Debt leverage ratio, maximum (as a percent)																							30.00%
Annual commitment fee on drawn balances (as a percent)																									0.25%	0.35%	0.45%
Annual commitment fee on undrawn balances (as a percent)																								0.125%