Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Investment securities available for sale
Cost	$ 4,362,725		$ 4,607,117
Gross Unrealized Gains	12,360		6,425
Gross Unrealized Losses	9,044		56,432
Fair Value	4,366,041		4,557,110
Cost	80,542		73,809
Gross unrealized gains	31,163	$ 19,010
Gross unrealized losses	216	$ 220
Fair value	111,489		92,599
Investment securities held to maturity
Cost	2,218,048		2,184,653
Held To Maturity Securities Accumulated Unrecognized Holding Gain	67,235
Held To Maturity Securities Accumulated Unrecognized Holding Loss	0
Fair Value	2,285,283		2,201,502
Cost	6,661,315		6,865,579
Gross unrealized gains	110,758		42,774
Gross unrealized losses	9,260		57,142
Fair value	6,762,813		6,851,211
U. S. Treasury
Investment securities available for sale
Cost	946,859		1,249,243
Gross Unrealized Gains	1,625		633
Gross Unrealized Losses	82		2,166
Fair Value	948,402		1,247,710
Agency Securities
Investment securities available for sale
Cost	533,269		257,252
Gross Unrealized Gains	1,724		222
Gross Unrealized Losses	189		639
Fair Value	534,804		256,835
Mortgage Backed Securities
Investment securities available for sale
Cost	2,734,696		2,956,793
Gross Unrealized Gains	7,420		5,309
Gross Unrealized Losses	8,418		52,763
Fair Value	2,733,698		2,909,339
Investment securities held to maturity
Cost	2,182,500		2,184,653
Held To Maturity Securities Accumulated Unrecognized Holding Gain	67,235		17,339
Held To Maturity Securities Accumulated Unrecognized Holding Loss	0		490
Fair Value	2,249,735		2,201,502
Corporate Bonds
Investment securities available for sale
Cost	147,901		143,829
Gross Unrealized Gains	1,591		261
Gross Unrealized Losses	355		864
Fair Value	149,137		$ 143,226
Other
Investment securities held to maturity
Cost	35,548
Held To Maturity Securities Accumulated Unrecognized Holding Gain	0
Held To Maturity Securities Accumulated Unrecognized Holding Loss	0
Fair Value	$ 35,548

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Debt Securities, Available-for-sale, Held-to-maturity and Equity Securities, Accumulated Gross Unrealized Gain, Before Tax

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