Shareholders' Equity (Reclassification from Accumulated Other Comprehensive Income) (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
(Losses) gains on available-for-sale securities, net	$ 18,000	$ (153,000)	$ (336,000)	$ (33,000)	$ (3,328,000)	$ 75,000	$ 2,000	$ 251,000	$ (504,000)	$ (3,000,000)	$ 4,895,000
Income before taxes	61,802,000	65,258,000	63,031,000	56,340,000	57,822,000	58,082,000	56,250,000	52,286,000	246,431,000	224,440,000	180,132,000
Income tax expense	(23,669,000)	(25,034,000)	(24,490,000)	(21,840,000)	(22,534,000)	(22,519,000)	(21,943,000)	(20,234,000)	(95,033,000)	(87,230,000)	(68,936,000)
Net income	38,133,000	40,224,000	38,541,000	34,500,000	35,288,000	35,563,000	34,307,000	32,052,000	151,398,000	137,210,000	111,196,000
Interest on junior subordinated debentures									8,079,000	11,369,000	12,616,000
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
(Losses) gains on available-for-sale securities, net									(504,000)	(3,000,000)
Income before taxes									(504,000)	(3,000,000)
Income tax expense									200,000	1,193,000
Net income									(304,000)	(1,807,000)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before taxes									(1,974,000)	(5,116,000)
Income tax expense									785,000	2,035,000
Net income									(1,189,000)	(3,081,000)
Interest on junior subordinated debentures									$ 1,974,000	$ 5,116,000