Stockholders' Equity - Other comprehensive income (loss) activity (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period			$ 844,002
Net current period other comprehensive income (loss)			6,347
Balance at the end of the period	$ 844,168		844,168
Accumulated Other Comprehensive Income(Loss)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(48,944)	$ (60,352)	(53,329)	$ (61,505)
Other comprehensive income (loss) before reclassification	1,341	2,473	5,087	2,467
Amount reclassified from accumulated other comprehensive income (loss)	621	371	1,260	1,530
Net current period other comprehensive income (loss)	1,962	2,844	6,347	3,997
Balance at the end of the period	(46,982)	(57,508)	(46,982)	(57,508)
Unrealized (gains) losses on investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(2,641)	(3,682)	(3,972)	(3,729)
Other comprehensive income (loss) before reclassification	722	1,554	2,145	1,550
Amount reclassified from accumulated other comprehensive income (loss)	(67)	(370)	(159)	(319)
Net current period other comprehensive income (loss)	655	1,184	1,986	1,231
Balance at the end of the period	(1,986)	(2,498)	(1,986)	(2,498)
Change in valuation allowance for unrealized gains (losses) on investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(1,120)	(3,211)	(3,388)	(3,240)
Other comprehensive income (loss) before reclassification	619	919	2,942	917
Amount reclassified from accumulated other comprehensive income (loss)	(39)	(219)	(94)	(188)
Net current period other comprehensive income (loss)	580	700	2,848	729
Balance at the end of the period	(540)	(2,511)	(540)	(2,511)
Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(45,183)	(53,459)	(45,969)	(54,536)
Amount reclassified from accumulated other comprehensive income (loss)	727	960	1,513	2,037
Net current period other comprehensive income (loss)	727	960	1,513	2,037
Balance at the end of the period	$ (44,456)	$ (52,499)	$ (44,456)	$ (52,499)