Securities Available for Sale and Held to Maturity - Contractual Maturities of Available for Sale and Held to Maturity Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Securities Available for Sale, Amortized Cost [Abstract]
Due in one year or less	$ 25,576
Due after one year through five years	24,719
Due after five years through ten years	3,557
Due after ten years	25,708
Total amortized cost, before mortgage-backed securities	79,560
Mortgage-backed securities	1,667,777
Securities Available for Sale, Amortized Cost	1,747,337	$ 1,854,529
Securities Available for Sale, Estimated Fair Value [Abstract]
Due in one year or less	25,599
Due after one year through five years	24,915
Due after five years through ten years	3,501
Due after ten years	24,947
Total fair value, before mortgage-backed securities	78,962
Mortgage-backed securities	1,644,483
Total	1,723,445	1,811,813
Securities Held to Maturity, Amortized Cost [Abstract]
Due in one year or less	170
Due after one year through five years	34,642
Due after five years through ten years	4,287
Due after ten years	0
Total amortized cost, before mortgage-backed securities	39,099
Mortgage-backed securities	845,220
Securities Held to Maturity, Amortized Cost	884,319	909,643
Securities Held to Maturity, Estimated Fair Value [Abstract]
Due in one year or less	170
Due after one year through five years	34,891
Due after five years through ten years	4,412
Due after ten years	0
Total fair value, before mortgage-backed securities	39,473
Mortgage-backed securities	835,385
Total	$ 874,858	$ 889,733