Subsequent Events (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		3 Months Ended										0 Months Ended										0 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 2013 Credit Agreement	Mar. 31, 2013 2013 Credit Agreement LIBOR Rate	Mar. 31, 2013 2013 Credit Agreement Base rate	Mar. 31, 2013 2013 Credit Agreement Prime rate	Mar. 31, 2013 2013 Credit Agreement Federal funds rate	Mar. 31, 2013 2013 Credit Agreement One-month LIBOR Rate	Mar. 31, 2013 Revolving credit facility	Mar. 31, 2013 Minimum 2013 Credit Agreement LIBOR Rate	Mar. 31, 2013 Minimum 2013 Credit Agreement Base rate	Mar. 31, 2013 Maximum 2013 Credit Agreement LIBOR Rate	Mar. 31, 2013 Maximum 2013 Credit Agreement Base rate	May 03, 2013 Subsequent events	May 03, 2013 Subsequent events 2013 Credit Agreement	May 03, 2013 Subsequent events Revolving credit facility item	May 03, 2013 Subsequent events Revolving credit facility LIBOR Rate	May 03, 2013 Subsequent events Revolving credit facility Federal funds rate	May 03, 2013 Subsequent events Revolving credit facility One-month LIBOR Rate	May 03, 2013 Subsequent events Minimum 2013 Credit Agreement	May 03, 2013 Subsequent events Minimum Revolving credit facility	May 03, 2013 Subsequent events Minimum Revolving credit facility LIBOR Rate	May 03, 2013 Subsequent events Minimum Revolving credit facility One-month LIBOR Rate	May 03, 2013 Subsequent events Maximum 2013 Credit Agreement	May 03, 2013 Subsequent events Maximum Revolving credit facility	May 03, 2013 Subsequent events Maximum Revolving credit facility LIBOR Rate	May 03, 2013 Subsequent events Maximum Revolving credit facility One-month LIBOR Rate
Subsequent events
Borrowing capacity														$ 500.0	$ 500.0
Period of revolving credit facility														5 years
Number of incremental term loan facilities															1
Amount of incremental credit facilities under specified circumstances														75.0
Additional amount of incremental credit facilities under specified circumstances																			25.0
Secured leverage ratio																							2.5
Maximum consolidated leverage ratio before reduction	0.0400												3.5
Maximum consolidated leverage ratio	0.0350												3.25
Reduction period of maximum consolidated leverage ratio	2 years												2 years
Minimum interest coverage ratio	0.0300												3
Interest rate margin (as a percent)						0.50%	1.00%		2.00%	1.00%	3.00%	2.00%					0.50%	1.00%			1.50%	0.50%			2.25%	1.25%
Interest rate of debts			LIBOR multiplied by a statutory reserve rate	Base Rate	Prime Rate	Federal funds rate	One-month LIBOR									LIBOR multiplied by a statutory reserve rate	Federal funds rate	one-month LIBOR
Interest rate at period end		3.3125%
Unused commitment fee on the revolving credit facility of the 2013 credit agreement (as a percent)								0.50%												0.375%				0.50%
Additional borrowing													$ 500