Investments (Unrealized Losses) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investment securities available for sale:
Fair Value, Less than 12 months	$ 1,817,847	$ 705,526
Unrealized Losses, Less than 12 months	5,841	3,943
Fair Value, 12 months or more	550,090	2,796,627
Unrealized Losses, Greater than 12 months	3,641	52,489
Fair Value, Total	2,367,937	3,502,153
Unrealized Losses, Total	9,482	56,432
Investment securities held to maturity
Unrealized Losses, Total	0
U. S. Treasury
Investment securities available for sale:
Fair Value, Less than 12 months	49,867	248,983
Unrealized Losses, Less than 12 months	2	113
Fair Value, 12 months or more	0	848,622
Unrealized Losses, Greater than 12 months	0	2,053
Fair Value, Total	49,867	1,097,605
Unrealized Losses, Total	2	2,166
Agency Securities
Investment securities available for sale:
Fair Value, Less than 12 months	323,470	115,273
Unrealized Losses, Less than 12 months	1,471	601
Fair Value, 12 months or more	66,278	2,310
Unrealized Losses, Greater than 12 months	341	38
Fair Value, Total	389,748	117,583
Unrealized Losses, Total	1,812	639
Mortgage-backed securities
Investment securities available for sale:
Fair Value, Less than 12 months	1,413,342	262,204
Unrealized Losses, Less than 12 months	4,165	2,387
Fair Value, 12 months or more	477,249	1,940,695
Unrealized Losses, Greater than 12 months	3,178	50,376
Fair Value, Total	1,890,591	2,202,899
Unrealized Losses, Total	7,343	52,763
Investment securities held to maturity
Fair Value, Less than 12 months		5,111
Unrealized Losses, Less than 12 months		181
Fair Value, 12 months or more		10,131
Unrealized Losses, 12 months or more		309
Fair Value, Total		15,242
Unrealized Losses, Total	0	490
Corporate bonds
Investment securities available for sale:
Fair Value, Less than 12 months		79,066
Unrealized Losses, Less than 12 months		842
Fair Value, 12 months or more		5,000
Unrealized Losses, Greater than 12 months		22
Fair Value, Total		84,066
Unrealized Losses, Total		864
Corporate Bonds
Investment securities available for sale:
Fair Value, Less than 12 months	31,168
Unrealized Losses, Less than 12 months	203
Fair Value, 12 months or more	6,563
Unrealized Losses, Greater than 12 months	122
Fair Value, Total	37,731
Unrealized Losses, Total	325	$ 864
Other
Investment securities held to maturity
Unrealized Losses, Total	$ 0

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