Accumulated Other Comprehensive Income Amounts reclassified from Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on sale of investment securities, net									$ 1,581	$ 552	$ 462
Income before income taxes	$ 37,338	$ 37,295	$ 31,799	$ 35,188	$ 28,087	$ 31,188	$ 29,870	$ 28,640	141,620	117,785	87,010
Income Tax Expense (Benefit)	(10,598)	(11,515)	(9,853)	(10,827)	(9,167)	(9,605)	(8,643)	(8,796)	(42,793)	(36,211)	(26,994)
Net income	$ 26,740	$ 25,780	$ 21,946	$ 24,361	$ 18,920	$ 21,583	$ 21,227	$ 19,844	98,827	81,574	60,016
Compensation and employee benefits									149,410	130,864	125,432
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									1,581	552	462
Income before income taxes									1,581	552	462
Income Tax Expense (Benefit)									(574)	(200)	(163)
Net income									1,007	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									(336)	(149)	(400)
Income Tax Expense (Benefit)									122	54	135
Net income									(214)	(95)	(265)
Compensation and employee benefits									$ (336)	$ (149)	$ (400)