Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 76,425	$ 75,074	$ 61,747
Other comprehensive (loss) income, net of tax
Unrealized net holding gains (losses) arising during the year (pre-tax amounts of ($3,159), $18,069, and ($41,059))	(1,930)	10,933	(24,794)
Reclassification adjustment for net gains related to securities available for sale included in net income (pre-tax amounts of $3,087, $92, and $1,426)	(1,886)	(56)	(861)
Amortization of unrealized net gains and losses related to the reclassification of available for sale investment securities to held to maturity (pre-tax amounts of ($1,311), ($421), and $0)	801	257	0
Amortization of prior service cost and actuarial gains (pre-tax amounts of $2,239, $75, and $2,790)	1,371	46	1,601
(Increase) Decrease in prior service cost and unrecognized actuarial loss (pre-tax amounts of ($6,144), ($19,245), and $21,923	(3,747)	(11,442)	13,169
Total other comprehensive (loss) income	(5,391)	(262)	(10,885)
Comprehensive income	$ 71,034	$ 74,812	$ 50,862