Accumulated Other Comprehensive Income Amounts reclassified from Accumulated Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	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]
Gain on sale of investment securities, net									$ 552	$ 462	$ 3,733
Income before income taxes	28,087	31,188	29,870	28,640	27,884	20,010	22,005	17,111	117,785	87,010	63,705
Income Tax Expense (Benefit)	(9,167)	(9,605)	(8,643)	(8,796)	(7,911)	(6,734)	(7,414)	(4,935)	(36,211)	(26,994)	(17,562)
Net income (loss)	18,920	21,583	21,227	19,844	19,973	13,276	14,591	12,176	81,574	60,016	46,143
Compensation and employee benefits									130,864	125,432	85,434
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]
Gain on sale of investment securities, net									552	462
Income before income taxes									552	462
Income Tax Expense (Benefit)									(200)	(163)
Net income (loss)									352	299
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes									(149)	(400)
Income Tax Expense (Benefit)									54	135
Net income (loss)									(95)	(265)
Compensation and employee benefits									$ (149)	$ (400)