Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income		$ 10,732	$ 24,467	$ 59,701	$ 61,326
Other comprehensive income (loss):
Unrealized holding (losses) gains on securities available-for-sale arising during the period		(1,238)	1,869	5,082	2,377
Reclassification adjustment for gains on realized income		(16)	(5)	(53)	(612)
Tax effect		439	(653)	(1,760)	(618)
Net unrealized (losses) gains on securities available-for-sale, net of tax		(815)	1,211	3,269	1,147
Pension plan remeasurement	[1]	(786)	0	(786)	0
Adjustment for pension and other postretirement benefits		2,895	1,087	1,742	3,261
Tax effect		(738)	(380)	(335)	(1,141)
Net adjustment for pension and other postretirement benefits		1,371	707	621	2,120
Other comprehensive income, net of tax		556	1,918	3,890	3,267
Total comprehensive income, net of tax		$ 11,288	$ 26,385	$ 63,591	$ 64,593

[1] Refer to Footnote 12, Pension and Other Postretirement Benefit Plans, for further information.