Investments (Unrealized Losses) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investment securities available for sale:
Fair Value, Less than 12 months	$ 487,273	$ 2,778,014
Unrealized Losses, Less than 12 months	6,001	16,025
Fair Value, 12 months or more	366,630	332,953
Unrealized Losses, Greater than 12 months	2,499	1,794
Fair Value, Total	853,903	3,110,967
Gross unrealized losses	8,500	17,819
Agency Securities
Investment securities available for sale:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	187,167	347,081
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,570	1,827
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	338,169	63,947
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,391	414
Fair Value, Total	525,336	411,028
Gross unrealized losses	4,961	2,241
Residential Mortgage Backed Securities
Investment securities available for sale:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	161,578	2,387,293
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	259	14,016
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	23,717	264,257
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	53	1,371
Fair Value, Total	185,295	2,651,550
Gross unrealized losses	312	15,387
Commercial Mortgage Backed Securities
Investment securities available for sale:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	56,703	35,926
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	434	59
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Fair Value, Total	56,703	35,926
Gross unrealized losses	434	59
Corporate bonds
Investment securities available for sale:
Fair Value, Total		12,463
Gross unrealized losses		132
Corporate Bonds
Investment securities available for sale:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	81,825	7,714
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,738	123
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	4,744	4,749
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	55	$ 9
Fair Value, Total	86,569
Gross unrealized losses	$ 2,793

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