Credit Agreement (Details) (Bank of America, USD $) In Millions, unless otherwise specified	1 Months Ended
	Jan. 31, 2011
Bank of America
Line of credit facility
Revolving credit facility	$ 100.0
Loans under credit agreement, interest rate base	LIBOR
Loans under credit agreement, additional interest rate base	PRIME
Loans under credit agreement, interest rate above base (as a percent)	1.50%
Covenant to maintain cash, cash equivalents and marketable securities in domestic deposit and securities accounts	400.0
Margined value of cash, cash equivalents and marketable securities	$ 100.0