Other Comprehensive Income/Loss (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Amounts reclassified from accumulated other comprehensive loss to the consolidated statements of income
Reclassification adjustment for gain (loss) on sales of securities	$ (141)	$ (2,057)	$ (8,477)
Reclassification adjustment for net actuarial loss	913	3,610	5,447
Reclassification adjustment for prior service cost			152
Income before income tax expense	122,195	104,342	80,971
Income tax effect	(26,279)	(37,749)	(27,880)
Net income	95,916	66,593	53,091
Amount reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive loss to the consolidated statements of income
Income before income tax expense	(962)	(1,766)
Income tax effect	339	621
Net income	(623)	(1,145)
Gain on sales of securities | Amount reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive loss to the consolidated statements of income
Reclassification adjustment for gain (loss) on sales of securities	141	2,057
Compensation and benefits | Amount reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive loss to the consolidated statements of income
Reclassification adjustment for net actuarial loss	(913)	(3,610)
Reclassification adjustment for prior service cost	$ (190)	$ (213)