Derivatives Derivative Effect on Accumulated Other Comprehensive Income (Details) (USD $)			6 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Interest rate swap [Member]	Jun. 30, 2012 Hedging instrument [Member] Interest rate swap [Member]	Jun. 30, 2011 Hedging instrument [Member] Interest rate swap [Member]	Jun. 30, 2012 Hedging instrument [Member] Interest rate swap [Member]	Jun. 30, 2011 Hedging instrument [Member] Interest rate swap [Member]	Jun. 30, 2012 Agency MBS [Member] Interest rate swap [Member]	Jun. 30, 2012 Cash [Member] Interest rate swap [Member]
Derivative [Line Items]
Balance as of beginning of period	$ (36,449,000)	$ (25,444,000)		$ (25,265,000)	$ 137,000	$ (25,444,000)	$ (2,820,000)
Change in fair value of interest rate swaps				(14,693,000)	(14,810,000)	(17,780,000)	(13,722,000)
Reclassification adjustment for amounts included in statement of income				3,509,000	3,072,000	6,775,000	4,941,000
Balance as of end of period	(36,449,000)	(25,444,000)		(36,449,000)	(11,601,000)	(36,449,000)	(11,601,000)
Existing losses in AOCI expected to be reclassified			15,139
Derivatives in net liability position			39,921
Collateral pledged for swaps in a net liability position								$ 43,328	$ 38