Shareholders' Equity Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ (72,585)		$ (36,174)		$ (10,967)
Reclassification of stranded accumulated other comprehensive loss to retained earnings related to tax reform					(6,550)
Transition adjustment of new accounting principle			2,862		(2,709)
Net change in unrealized gain (loss)	241,047		(48,010)		(26,152)
Reclassification adjustments included in earnings: [Abstract]
Loss (gain) on available for sale securities, net	(5,597)		2,801		(4,428)
Other comprehensive income (loss), before income taxes	235,450		(45,209)		(30,580)
Federal and state income taxes	57,942	[1]	(11,507)	[1]	(11,923)	[2]
Other comprehensive income (loss), net of income taxes	177,508		(33,702)		(18,657)
Ending balance	104,923		(72,585)		(36,174)
Unrealized Gain (Loss) on Available for Sale Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(70,999)		(35,385)		(9,087)
Reclassification of stranded accumulated other comprehensive loss to retained earnings related to tax reform					(6,408)
Transition adjustment of new accounting principle					(2,709)
Net change in unrealized gain (loss)	239,017		(46,941)		(28,170)
Reclassification adjustments included in earnings: [Abstract]
Loss (gain) on available for sale securities, net	(5,597)		2,801		(4,428)
Other comprehensive income (loss), before income taxes	233,420		(44,140)		(32,598)
Federal and state income taxes	57,425	[1]	(11,235)	[1]	(12,708)	[2]
Other comprehensive income (loss), net of income taxes	175,995		(32,905)		(19,890)
Ending balance	104,996		(70,999)		(35,385)
Unrealized Gain (Loss) on Employee Benefit Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,586)		(789)		(1,880)
Reclassification of stranded accumulated other comprehensive loss to retained earnings related to tax reform					(142)
Transition adjustment of new accounting principle					0
Net change in unrealized gain (loss)	2,030		(1,069)		2,018
Reclassification adjustments included in earnings: [Abstract]
Loss (gain) on available for sale securities, net	0		0		0
Other comprehensive income (loss), before income taxes	2,030		(1,069)		2,018
Federal and state income taxes	517	[1]	(272)	[1]	785	[2]
Other comprehensive income (loss), net of income taxes	1,513		(797)		1,233
Ending balance	$ (73)		$ (1,586)		$ (789)

[1] Calculated using a 25 percent blended federal and state statutory tax rate.

[2] Calculated using a 39 percent blended federal and state statutory tax rate.