Fair Value Measurements - Carrying Amounts and Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investment securities:
Held-to-maturity	$ 575,502	$ 618,672
Level 3 | Nonrecurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	79,649	64,130
Level 3 | Nonrecurring | Impaired loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	76,186	63,247
Carrying Amount | Level 2
Investment securities:
Held-to-maturity	565,546	623,599
Financial liabilities:
Time deposits	4,360,014	4,074,248
Collateralized customer deposits	291,671	382,687
Short-term borrowings	2,615,000	2,035,000
Long-term borrowings	426,069	426,002
Carrying Amount | Level 3
Investment securities:
Held-to-maturity	500	500
Net loans	15,745,936	15,159,795
Fair Value | Level 2
Investment securities:
Held-to-maturity	575,042	618,232
Financial liabilities:
Time deposits	4,354,554	4,041,212
Collateralized customer deposits	291,513	382,370
Short-term borrowings	2,614,777	2,034,719
Long-term borrowings	431,043	423,258
Fair Value | Level 3
Investment securities:
Held-to-maturity	460	440
Net loans	$ 15,717,256	$ 14,907,789