Stockholders' Equity - Other comprehensive income (loss) activity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	$ 844,002
Net current period other comprehensive income (loss)	4,385
Balance at the end of the period	848,884
Accumulated Other Comprehensive Income(Loss)
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(53,329)	$ (61,505)
Other comprehensive income (loss) before reclassification	3,747	(5)
Amount reclassified from accumulated other comprehensive income (loss)	638	1,158
Net current period other comprehensive income (loss)	4,385	1,153
Balance at the end of the period	(48,944)	(60,352)
Unrealized (gains) losses on investment securities
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(3,972)	(3,729)
Other comprehensive income (loss) before reclassification	1,423	(4)
Amount reclassified from accumulated other comprehensive income (loss)	(92)	51
Net current period other comprehensive income (loss)	1,331	47
Balance at the end of the period	(2,641)	(3,682)
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	(3,388)	(3,240)
Other comprehensive income (loss) before reclassification	2,324	(1)
Amount reclassified from accumulated other comprehensive income (loss)	(56)	30
Net current period other comprehensive income (loss)	2,268	29
Balance at the end of the period	(1,120)	(3,211)
Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(45,969)	(54,536)
Amount reclassified from accumulated other comprehensive income (loss)	786	1,077
Net current period other comprehensive income (loss)	786	1,077
Balance at the end of the period	$ (45,183)	$ (53,459)