Accumulated Other Comrehensive Loss (Components of AOCI) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss	$ (27,276)	$ 18,292
Net current period other comprehensive income	138,986	108,464	$ (62,842)
Unrealized gains on securities available for sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Stockholders' equity	5,792	(38,505)
Net unrealized gains (losses) arising during period	119,357	49,776
Amounts reclassified from accumulated other comprehensive loss	(46,395)	(5,479)
Net current period other comprehensive income	72,962	44,297
Stockholders' equity	78,754	5,792	(38,505)
Unrealized gains on securities available for sale transferred from (to) held to maturity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Stockholders' equity	0	(71,149)
Net unrealized gains (losses) arising during period	4,538	55,834
Amounts reclassified from accumulated other comprehensive loss	(381)	15,315
Net current period other comprehensive income	4,157	71,149
Stockholders' equity	4,157	0	(71,149)
Defined Benefit Pension Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Stockholders' equity	(132,515)	(125,533)
Net unrealized gains (losses) arising during period	42,367	(15,438)
Amounts reclassified from accumulated other comprehensive loss	19,500	8,456
Net current period other comprehensive income	61,867	(6,982)
Stockholders' equity	(70,648)	(132,515)	(125,533)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Stockholders' equity	(126,723)	(235,187)
Net unrealized gains (losses) arising during period	166,262	90,172
Amounts reclassified from accumulated other comprehensive loss	(27,276)	18,292
Net current period other comprehensive income	138,986	108,464	(62,842)
Stockholders' equity	$ 12,263	$ (126,723)	$ (235,187)