Stockholders' Equity - Other comprehensive income (loss) activity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 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)			3,486
Balance at the end of the period	$ 845,477		845,477
Accumulated Other Comprehensive Income(Loss)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(57,508)	$ (51,731)	(61,505)	$ (50,443)
Other comprehensive income (loss) before reclassification	1,460	(3,427)	3,923	(3,807)
Amount reclassified from accumulated other comprehensive income (loss)	(1,971)	2,994	(437)	2,086
Net current period other comprehensive income (loss)	(511)	(433)	3,486	(1,721)
Balance at the end of the period	(58,019)	(52,164)	(58,019)	(52,164)
Unrealized (gains) losses on investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(2,498)	(2,713)	(3,729)	(727)
Other comprehensive income (loss) before reclassification	1,660	(2,165)	3,207	(2,387)
Amount reclassified from accumulated other comprehensive income (loss)	(1,871)	1,330	(2,187)	(434)
Net current period other comprehensive income (loss)	(211)	(835)	1,020	(2,821)
Balance at the end of the period	(2,709)	(3,548)	(2,709)	(3,548)
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	(2,511)	(2,649)	(3,240)	(1,471)
Other comprehensive income (loss) before reclassification	1,022	(1,262)	1,938	(1,420)
Amount reclassified from accumulated other comprehensive income (loss)	(1,155)	776	(1,342)	(244)
Net current period other comprehensive income (loss)	(133)	(486)	596	(1,664)
Balance at the end of the period	(2,644)	(3,135)	(2,644)	(3,135)
Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(52,499)	(46,369)	(54,536)	(48,245)
Other comprehensive income (loss) before reclassification	(1,222)		(1,222)
Amount reclassified from accumulated other comprehensive income (loss)	1,055	888	3,092	2,764
Net current period other comprehensive income (loss)	(167)	888	1,870	2,764
Balance at the end of the period	$ (52,666)	$ (45,481)	$ (52,666)	$ (45,481)