Other Comprehensive Income (Components of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at December 31,	$ 1,413,840	$ 1,358,980
Current - period other comprehensive income (loss)	11,117	7,252
Balance at March 31,	1,412,589	1,373,816
Unrealized Gains on Available for Sale Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at December 31,	8,746	(9,605)
Current - period other comprehensive income (loss)	16,746	7,578
Balance at March 31,	25,492	(2,027)
Post-retirement obligations
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at December 31,	(5,240)	(3,625)
Current - period other comprehensive income (loss)	84	(12)
Balance at March 31,	(5,156)	(3,637)
Unrealized gains (losses) on derivatives (cash flow hedges)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at December 31,	315	894
Current - period other comprehensive income (loss)	(5,713)	(314)
Balance at March 31,	(5,398)	580
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at December 31,	3,821	(12,336)
Current - period other comprehensive income (loss)	11,117	7,252
Balance at March 31,	$ 14,938	$ (5,084)