Debt Securities Available for Sale and Debt Securities Held to Maturity - Maturities of securities portfolios (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment securities available for sale, amortized cost [Abstract]
Maturing within one year	$ 250,930
Maturing after one but within five years	2,338,348
Maturing after five but within ten years	1,492,547
Maturing after ten years	827,875
Total single date maturities	4,909,700
Mortgage-backed securities and collateralized mortgage obligations	6,771,355
Available-for-sale securities, amortized cost basis	11,681,055	$ 12,411,689
Investment securities available for sale, fair value [Abstract]
Maturing within one year	250,048
Maturing after one but within five years	2,275,290
Maturing after five but within ten years	1,473,277
Maturing after ten years	797,370
Total single date maturities	4,795,985
Mortgage-backed securities and collateralized mortgage obligations	6,638,167
Investment securities available for sale	11,434,152	12,219,632
Held-to-maturity securities, amortized cost [Abstract]
Maturing within one year	107,667
Maturing after one but within five years	182,436
Maturing after five but within ten years	206,061
Maturing after ten years	469,897
Total single date maturities	966,061
Collateralized mortgage obligations	1,009,668
Total	1,975,729	1,046,093
Held-to-maturity securities, fair value [Abstract]
Maturing within one year	107,742
Maturing after one but within five years	183,167
Maturing after five but within ten years	206,733
Maturing after ten years	472,210
Total single date maturities	969,852
Collateralized mortgage obligations	998,144
Total Investments Securities Held to Maturity	$ 1,967,996	$ 1,040,543