Fair Value Measurements - Carrying Amounts and Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment securities:
Held-to-maturity	$ 663,621	$ 662,906
Nonrecurring
Financial liabilities:
Impaired loans recorded at fair value on a nonrecurring basis	56,829	73,518
Nonrecurring | Impaired loans
Financial liabilities:
Impaired loans recorded at fair value on a nonrecurring basis	54,060	70,619
Level 3 | Nonrecurring
Financial liabilities:
Impaired loans recorded at fair value on a nonrecurring basis	56,829	73,518
Level 3 | Nonrecurring | Impaired loans
Financial liabilities:
Impaired loans recorded at fair value on a nonrecurring basis	54,060	70,619
Carrying Amount | Level 2
Investment securities:
Held-to-maturity	676,347	676,593
Financial liabilities:
Time deposits	3,360,113	3,217,207
Collateralized customer deposits	490,107	415,236
Short-term borrowings	2,050,000	2,000,000
Long-term borrowings	372,908	372,882
Carrying Amount | Level 3
Investment securities:
Held-to-maturity	500	500
Net loans	14,123,985	14,063,380
Fair Value | Level 2
Investment securities:
Held-to-maturity	663,196	662,516
Financial liabilities:
Time deposits	3,327,989	3,225,847
Collateralized customer deposits	488,683	415,236
Short-term borrowings	2,049,162	1,999,137
Long-term borrowings	368,915	367,984
Fair Value | Level 3
Investment securities:
Held-to-maturity	425	390
Net loans	$ 13,755,579	$ 14,114,545