Fair Value Measurements, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	$ 3,283,456	$ 3,946,941
Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	0	999
Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	3,283,456	3,945,943
Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Trading assets	4,869	2,208
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,903	2,072
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | US Treasury Securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	0	999
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Other Debt And Other Equity Securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	1,621	1,568
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Total investment securities available for sale
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	0	999
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | GNMA
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	1,331,633	2,128,531
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Private-label
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	814	1,003
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | FFELP asset backed securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	273,137	297,360
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Debt And Other Equity Securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	3,000	3,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Total investment securities available for sale
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	3,283,456	3,945,943
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Residential Loans Held For Sale Member
Assets, Fair Value Disclosure [Abstract]
Trading assets	129,303	64,321
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Interest rate-related instruments — customer and mirror
Assets, Fair Value Disclosure [Abstract]
Trading assets	77,264	52,796
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	15,013	52,653
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign currency exchange forwards
Assets, Fair Value Disclosure [Abstract]
Trading assets	2,683	721
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	2,425	675
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commodity contracts
Assets, Fair Value Disclosure [Abstract]
Trading assets	22,498	35,426
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	22,076	34,340
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Purchased options (time deposit)
Assets, Fair Value Disclosure [Abstract]
Trading assets	126	109
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	126	109
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Interest rate lock commitments (mortgage)
Assets, Fair Value Disclosure [Abstract]
Trading assets	4,869	2,208
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Forward commitments (mortgage)
Liabilities, Fair Value Disclosure [Abstract]
Trading liabilities	1,903	2,072
Designated as Hedging Instrument
Assets, Fair Value Disclosure [Abstract]
Trading assets	500,000
Designated as Hedging Instrument | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Interest Rate Fair Value Hedge Asset at Fair Value	69	0
Liabilities, Fair Value Disclosure [Abstract]
Interest Rate Fair Value Hedge Liability at Fair Value	0	40
FNMA/FHLMC | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | FNMA / FHLMC
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	183,722	295,252
FNMA/FHLMC | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commercial mortgage-related securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	21,102	0
GNMA | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commercial mortgage-related securities
Assets, Fair Value Disclosure [Abstract]
Investment securities available for sale	$ 1,470,048	$ 1,220,797