Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 78,409	$ 76,425	$ 75,074
Other comprehensive income, net of tax:
Unrealized net holding (losses) gains arising during the year (pre-tax amounts of $(8,618), $(3,159), and $18,069)	(5,265)	(1,930)	10,933
Reclassification adjustment for net losses (gains) related to securities available for sale included in net income (pre-tax amounts of $644, $(3,087), and $(92))	393	(1,886)	(56)
Unrealized gains on derivates (cash flow hedges) (pre-tax amounts of $2,901)	1,772	0	0
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 $(1,094), $(1,311), and $(421))	668	801	257
Amortization of prior service cost and actuarial gains (pre-tax amounts of $2,370, $2,239, and $75)	1,421	1,371	46
Decrease (increase) in unrecognized actuarial loss (pre-tax amounts of $3,154, $(6,144), and $(19,245))	1,909	(3,747)	(11,442)
Total other comprehensive income (loss)	898	(5,391)	(262)
Comprehensive income	$ 79,307	$ 71,034	$ 74,812