Securities - Investment Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Held to Maturity, amortized cost, due in one year or less	$ 48,722
Held to Maturity, amortized cost, due after one year through five years	117,183
Held to Maturity, amortized cost, due after five years through ten years	68,317
Held to Maturity, amortized cost, due after ten years	4,516
Held to Maturity, amortized cost, subtotal	238,738
Held to Maturity, amortized cost, mortgage-backed securities and collateralized mortgage obligations	8,406,425
Held to Maturity, Amortized Cost	8,645,163	$ 9,324,811
Held to Maturity, fair value, due in one year or less	48,797
Held to Maturity, fair value, due after one year through five years	118,201
Held to Maturity, fair value, due after five years through ten years	73,134
Held to Maturity, fair value, due after ten years	4,590
Held to Maturity, fair value, subtotal	244,722
Held to Maturity, fair value, mortgage-backed securities and collateralized mortgage obligations	8,411,687
Held to Maturity, fair value, total	8,656,409	9,081,236
Available for Sale, amortized cost, due in one year or less	165
Available for Sale, amortized cost, due after one year through five years	304
Available for Sale, amortized cost, subtotal	469
Available for Sale, amortized cost, mortgage-backed securities and collateralized mortgage obligations	304,697
Available for Sale, Amortized Cost	305,166	83,763
Available for Sale, fair value, due in one year or less	165
Available for Sale, fair value, due after one year through five years	305
Available for Sale, fair value, subtotal	470
Available for Sale, fair value, mortgage-backed securities and collateralized mortgage obligations	306,307
Available for Sale securities, at fair value	$ 306,777	$ 84,155