Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of comprehensive income
Net income	$ 32,776	$ 48,074	$ 44,294	$ 70,477	$ 79,628	$ 84,348	$ 67,213	$ 42,182	$ 195,621	$ 273,371	$ 149,279
Other comprehensive income:
Unrealized gain on held-to-maturity investment portfolio at time of transfer to available-for-sale, net of tax											3,030
Unrealized gain (loss) on available-for-sale securities, net of tax									11,315	(727)	2,346
Total change									11,315	(727)	5,376
Comprehensive income									$ 206,936	$ 272,644	$ 154,655