Fair Value Measurements - Carrying Amounts and Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment securities:
Held-to-maturity	$ 592,298	$ 662,906
Nonrecurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	52,081	73,518
Nonrecurring | Impaired loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	50,394	70,619
Level 3 | Nonrecurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	52,081	73,518
Level 3 | Nonrecurring | Impaired loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	50,394	70,619
Carrying Amount | Level 2
Investment securities:
Held-to-maturity	602,187	676,593
Financial liabilities:
Time deposits	3,878,686	3,217,207
Collateralized customer deposits	378,938	415,236
Short-term borrowings	2,095,000	2,000,000
Long-term borrowings	330,956	372,882
Carrying Amount | Level 3
Investment securities:
Held-to-maturity	500	500
Net loans	14,479,678	14,063,380
Fair Value | Level 2
Investment securities:
Held-to-maturity	591,873	662,516
Financial liabilities:
Time deposits	3,844,389	3,225,847
Collateralized customer deposits	378,134	415,236
Short-term borrowings	2,094,567	1,999,137
Long-term borrowings	328,495	367,984
Fair Value | Level 3
Investment securities:
Held-to-maturity	425	390
Net loans	$ 14,082,333	$ 14,114,545