Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Statements of Comprehensive Income [Abstract]
Net Income	$ 20,758	$ 19,444	$ 18,981	$ 16,333	$ 14,348	$ (19,048)	$ 11,886	$ 10,285	$ 75,516	$ 17,471	$ 42,330
Unrealized gains on available-for-sale securities									31,617	63,190	6,263
Reclassification adjustment for gains included in net income									0	(346)	(4,822)
Net unrealized gains on securities									31,617	62,844	1,441
Tax effect									(12,300)	(24,444)	(565)
Net of tax amount									19,317	38,400	876
Unrealized losses on derivatives used for cash flow hedges									(7,926)	(8,906)	0
Tax effect									3,084	3,465	0
Net of tax amount									(4,842)	(5,441)	0
Total other comprehensive income, net of tax									14,475	32,959	876
Total Comprehensive Income									$ 89,991	$ 50,430	$ 43,206