Stockholders' Equity - Other comprehensive income (loss) activity (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period			$ 846,455
Net current period other comprehensive income (loss)			3,997
Balance at the end of the period	$ 817,559		817,559
Accumulated Other Comprehensive Income(Loss)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(60,352)	$ (47,690)	(61,505)	$ (50,443)
Other comprehensive income (loss) before reclassification	2,473	(4,516)	2,467	(380)
Amount reclassified from accumulated other comprehensive income (loss)	371	475	1,530	(908)
Net current period other comprehensive income (loss)	2,844	(4,041)	3,997	(1,288)
Balance at the end of the period	(57,508)	(51,731)	(57,508)	(51,731)
Unrealized (gains) losses on investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(3,682)	421	(3,729)	(727)
Other comprehensive income (loss) before reclassification	1,554	(2,845)	1,550	(222)
Amount reclassified from accumulated other comprehensive income (loss)	(370)	(289)	(319)	(1,764)
Net current period other comprehensive income (loss)	1,184	(3,134)	1,231	(1,986)
Balance at the end of the period	(2,498)	(2,713)	(2,498)	(2,713)
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	(3,211)	(809)	(3,240)	(1,471)
Other comprehensive income (loss) before reclassification	919	(1,671)	917	(158)
Amount reclassified from accumulated other comprehensive income (loss)	(219)	(169)	(188)	(1,020)
Net current period other comprehensive income (loss)	700	(1,840)	729	(1,178)
Balance at the end of the period	(2,511)	(2,649)	(2,511)	(2,649)
Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(53,459)	(47,302)	(54,536)	(48,245)
Amount reclassified from accumulated other comprehensive income (loss)	960	933	2,037	1,876
Net current period other comprehensive income (loss)	960	933	2,037	1,876
Balance at the end of the period	$ (52,499)	$ (46,369)	$ (52,499)	$ (46,369)