Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Consolidated Statements of Comprehensive Income (unaudited) [Abstract]
Net income	$ 19,909	$ 19,281	$ 38,800	$ 37,447
Other comprehensive income (loss), net of tax:
Unrealized net holding gains (losses) arising during the period (pre-tax amounts of $3,730, ($5,277), $16,941 and ($434))	2,278	(3,223)	10,350	(266)
Reclassification adjustment for net gains related to securities available for sale included in net income (pre-tax amounts of $1, $26, $30 and $40)	0	(16)	(19)	(24)
Unrealized losses on derivatives (cash flow hedges) (pre-tax amounts of ($62), $0, ($62) and $0)	(38)	0	(38)	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 $280, $336, $576 and $671)	171	205	352	410
Pension and other benefits:
Amortization of prior service cost and actuarial gains (pre-tax amounts of $513, $561, $1,024 and $1,122)	313	342	626	684
Total other comprehensive income (loss)	2,724	(2,692)	11,271	804
Comprehensive income	$ 22,633	$ 16,589	$ 50,071	$ 38,251