Debt Securities Available for Sale and Debt Securities Held to Maturity - Maturities of securities portfolios (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Investment securities available for sale, amortized cost [Abstract]
Maturing within one year	$ 325,189
Maturing after one but within five years	1,393,467
Maturing after five but within ten years	20,904
Maturing after ten years	437,881
Total single date maturities	2,177,441
Mortgage-backed securities and collateralized mortgage obligations	3,453,991
Available-for-sale securities, amortized cost	5,631,432	$ 7,251,523
Investment securities available for sale, fair value [Abstract]
Maturing within one year	325,784
Maturing after one but within five years	1,446,864
Maturing after five but within ten years	21,824
Maturing after ten years	423,980
Total single date maturities	2,218,452
Mortgage-backed securities and collateralized mortgage obligations	3,546,740
Total	5,765,192	7,235,305
Held-to-maturity securities, amortized cost [Abstract]
Maturing within one year	25,500
Maturing after one but within five years	1,430,336
Maturing after five but within ten years	254,879
Maturing after ten years	163,453
Total single date maturities	1,874,168
Collateralized mortgage obligations	6,834,286
Held-to-maturity securities, amortized cost	8,708,454	6,797,046
Held-to-maturity securities, fair value [Abstract]
Maturing within one year	25,497
Maturing after one but within five years	1,556,878
Maturing after five but within ten years	257,822
Maturing after ten years	161,516
Total single date maturities	2,001,713
Collateralized mortgage obligations	7,076,585
Total	$ 9,078,298	$ 6,921,158