Securities - Amortized Cost and Estimated Fair Value of Investments in Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Available for sale, Amortized Cost
Due in one year or less	$ 1,263
Due after one year through five years	9,177
Due after five years through ten years	33,327
Due after ten years	594,532
Total available-for-sale investment securities	638,299
Mortgage-backed securities:	1,541,510
Amortized Cost	2,179,809	$ 2,013,485
Available for sale, Fair Value
Due in one year or less	1,267
Due after one year through five years	9,284
Due after five years through ten years	34,725
Due after ten years	617,773
Total available-for-sale investment securities	663,049
Mortgage-backed securities:	1,577,332
Total	2,240,381	1,989,436
Held to maturity, Amortized Cost
Due in one year or less	115
Due after one year through five years	1,668
Due after five years through ten years	1,112
Due after ten years	0
Total held-to-maturity investment securities	2,895
Mortgage-backed securities	138,060
Amortized cost	140,955	162,931
Held to maturity, Fair Value
Due in one year or less	116
Due after one year through five years	1,683
Due after five years through ten years	1,129
Due after ten years	0
Total held-to-maturity investment securities	2,928
Mortgage-backed securities	144,112
Total	$ 147,040	$ 159,781