Debt Securities Available for Sale and Debt Securities Held to Maturity - Maturities of securities portfolios (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investment securities available for sale, amortized cost [Abstract]
Maturing within one year	$ 425,274
Maturing after one but within five years	1,646,452
Maturing after five but within ten years	21,114
Maturing after ten years	442,005
Total single date maturities	2,534,845
Mortgage-backed securities and collateralized mortgage obligations	3,714,385
Available-for-sale securities, amortized cost	6,249,230	$ 7,251,523
Investment securities available for sale, fair value [Abstract]
Maturing within one year	426,343
Maturing after one but within five years	1,706,765
Maturing after five but within ten years	21,833
Maturing after ten years	416,089
Total single date maturities	2,571,030
Mortgage-backed securities and collateralized mortgage obligations	3,773,786
Total	6,344,816	7,235,305
Held-to-maturity securities, amortized cost [Abstract]
Maturing within one year	49,190
Maturing after one but within five years	1,391,311
Maturing after five but within ten years	272,817
Maturing after ten years	169,456
Total single date maturities	1,882,774
Collateralized mortgage obligations	5,995,384
Held-to-maturity securities, amortized cost	7,878,158	6,797,046
Held-to-maturity securities, fair value [Abstract]
Maturing within one year	50,040
Maturing after one but within five years	1,519,329
Maturing after five but within ten years	274,289
Maturing after ten years	164,640
Total single date maturities	2,008,298
Collateralized mortgage obligations	6,259,095
Total	$ 8,267,393	$ 6,921,158