Debt (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Maximum borrowing capacity	$ 150,000		$ 150,000
Number of credit facilities			2
Revolving credit facility, maturity date			July 1, 2014
Percentage of daily amounts of commitment paid as facility fee			0.25%
Borrowings under the credit agreement	150,000		150,000
Interest rate under credit agreement	0.61%		0.61%
Amount of investments and cash equivalents served as collateral for the secured borrowings	157,950		157,950
Interest expense	507	514	1,064	934
Revolving Credit Facility, One [Member]
Maximum borrowing capacity	50,000		50,000
Revolving Credit Facility, Two [Member]
Maximum borrowing capacity	$ 100,000		$ 100,000