Investments (Unrealized Losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investment securities available for sale:
Less than 12 months, fair value	$ 2,778,014	$ 705,526
Less than 12 months, unrealized losses	16,025	3,943
Longer than 12 months, fair value	332,953	2,796,627
Longer than 12 months, unrealized losses	1,794	52,489
Total fair value	3,110,967	3,502,153
Total unrealized losses	17,819	56,432
U. S. Treasury
Investment securities available for sale:
Less than 12 months, fair value		248,983
Less than 12 months, unrealized losses		113
Longer than 12 months, fair value		848,622
Longer than 12 months, unrealized losses		2,053
Total fair value		1,097,605
Total unrealized losses		2,166
Government Agency
Investment securities available for sale:
Less than 12 months, fair value	347,081	115,273
Less than 12 months, unrealized losses	1,827	601
Longer than 12 months, fair value	63,947	2,310
Longer than 12 months, unrealized losses	414	38
Total fair value	411,028	117,583
Total unrealized losses	2,241	639
Residential Mortgage Backed Securities
Investment securities available for sale:
Less than 12 months, fair value	2,387,293	262,204
Less than 12 months, unrealized losses	14,016	2,387
Longer than 12 months, fair value	264,257	1,940,695
Longer than 12 months, unrealized losses	1,371	50,376
Total fair value	2,651,550	2,202,899
Total unrealized losses	15,387	52,763
Investment securities held to maturity:
Less than 12 months, fair value		5,111
Less than 12 months, unrealized losses		181
Longer than 12 months, fair value		10,131
Longer than 12 months, unrealized losses		309
Total fair value		15,242
Total unrealized losses		490
Commercial Mortgage Backed Securities
Investment securities available for sale:
Less than 12 months, fair value	35,926
Less than 12 months, unrealized losses	59
Longer than 12 months, fair value	0
Longer than 12 months, unrealized losses	0
Total fair value	35,926
Total unrealized losses	59
Corporate Bonds
Investment securities available for sale:
Less than 12 months, fair value	7,714	79,066
Less than 12 months, unrealized losses	123	842
Longer than 12 months, fair value	4,749	5,000
Longer than 12 months, unrealized losses	9	22
Total fair value	12,463	84,066
Total unrealized losses	$ 132	$ 864

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