Securities Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value (Details) $ in Thousands	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	166	490
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 1,381,046	$ 700,064
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	978,329	5,631,803
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,359,375	6,331,867
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4,872	1,615
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	8,784	136,642
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 13,656	$ 138,257
U.S. Treasury [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	0	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	493
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0	493
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 0	$ 3
Residential agency mortgage-backed securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	108	289
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 931,248	$ 510,824
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	753,448	3,641,370
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,684,696	4,152,194
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3,260	1,158
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	7,848	96,606
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 11,108	$ 97,764
Commercial agency mortgage-backed securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	57	197
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 449,798	$ 179,258
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	224,409	1,969,504
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	674,207	2,148,762
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,612	394
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	908	39,969
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 2,520	$ 40,363
Other debt securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	1	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 0	$ 9,982
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	472	20,436
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	472	30,418
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	63
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	28	64
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 28	$ 127