Securities Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value (Details) $ in Thousands	Jun. 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	205	490
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 707,576	$ 700,064
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,913,442	5,631,803
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,621,018	6,331,867
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,793	1,615
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	17,138	136,642
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 18,931	$ 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	1	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	498	493
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	498	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	1	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 1	$ 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	125	289
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 267,481	$ 510,824
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,317,121	3,641,370
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,584,602	4,152,194
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	898	1,158
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	14,564	96,606
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 15,462	$ 97,764
Commercial mortgage-backed securities guaranteed by U.S. government agencies [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	78	197
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 440,095	$ 179,258
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	595,351	1,969,504
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,035,446	2,148,762
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	895	394
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,545	39,969
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 3,440	$ 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