Accumulated Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Components of Accumulated Other Comprehensive Income
Unrealized gains on available-for-sale securities	$ 95,328	$ 63,711
Tax effect	(37,083)	(24,783)
Net of tax amount	58,245	38,928
Unrealized losses on derivatives used for cash flow hedges	(16,832)	(8,906)
Tax effect	6,549	3,465
Net of tax amount	(10,283)	(5,441)
Total accumulated other comprehensive income	$ 47,962	$ 33,487