Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]
Balance at the beginning of the period	$ 1,358,980	$ 1,298,661	$ 1,251,781
Current period change in other comprehensive income (loss)	16,157	(4,687)	(2,673)
Balance at the end of the period	1,413,840	1,358,980	1,298,661
Unrealized Losses on Available for Sale Debt Securities
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]
Balance at the beginning of the period	(9,605)	(3,292)
Current period change in other comprehensive income (loss)	18,351	(6,129)
Effect of adopting ASU No. 2016-02	0	(184)
Balance at the end of the period	8,746	(9,605)	(3,292)
Post-Retirement Obligations
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]
Balance at the beginning of the period	(3,625)	(4,846)
Current period change in other comprehensive income (loss)	(1,615)	1,221
Effect of adopting ASU No. 2016-02	0	0
Balance at the end of the period	(5,240)	(3,625)	(4,846)
Unrealized gains (losses) on Derivatives (cash flow hedges)
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]
Balance at the beginning of the period	894	673
Current period change in other comprehensive income (loss)	(579)	221
Effect of adopting ASU No. 2016-02	0	0
Balance at the end of the period	315	894	673
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]
Balance at the beginning of the period	(12,336)	(7,465)	(3,397)
Current period change in other comprehensive income (loss)	16,157	(4,687)	(2,673)
Effect of adopting ASU No. 2016-02	0	(184)
Balance at the end of the period	$ 3,821	$ (12,336)	$ (7,465)