Debt Securities Available for Sale and Debt Securities Held to Maturity - Maturities of securities portfolios (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investment securities available for sale, amortized cost [Abstract]
Maturing within one year	$ 325,088
Maturing after one but within five years	1,400,509
Maturing after five but within ten years	6,417
Maturing after ten years	422,656
Total single date maturities	2,154,670
Mortgage-backed securities and collateralized mortgage obligations	3,747,932
Available-for-sale securities, amortized cost	5,902,602	$ 7,251,523
Investment securities available for sale, fair value [Abstract]
Maturing within one year	325,409
Maturing after one but within five years	1,449,556
Maturing after five but within ten years	6,514
Maturing after ten years	410,166
Total single date maturities	2,191,645
Mortgage-backed securities and collateralized mortgage obligations	3,836,427
Total	6,028,072	7,235,305
Held-to-maturity securities, amortized cost [Abstract]
Maturing within one year	24,474
Maturing after one but within five years	1,419,845
Maturing after five but within ten years	177,387
Maturing after ten years	152,340
Total single date maturities	1,774,046
Collateralized mortgage obligations	7,669,990
Held-to-maturity securities, amortized cost	9,444,036	6,797,046
Held-to-maturity securities, fair value [Abstract]
Maturing within one year	24,517
Maturing after one but within five years	1,541,388
Maturing after five but within ten years	185,275
Maturing after ten years	151,832
Total single date maturities	1,903,012
Collateralized mortgage obligations	7,876,183
Total	$ 9,779,195	$ 6,921,158