Fair Value - Financial Instruments not Required to be Carried at Fair Value and Off-Balance Sheet Commitments (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Lease and lease commitments excluded
Financing leases	$ 187,200	$ 198,700
Financing lease commitments	0	100
Book Value
Financial assets:
Short-term financial assets	1,175,820	2,650,195
Loans	11,209,378	10,523,351
Financial liabilities:
Deposits	16,965,527	16,061,924
Short-term borrowings	9,151	216,151
Book Value | Commitments to extend credit
Off-balance sheet financial instruments:
Off-balance sheet financial instrument amount	21,534	25,113
Book Value | Standby letters of credit
Off-balance sheet financial instruments:
Off-balance sheet financial instrument amount	1,533	2,122
Book Value | Commercial Letters of credit
Off-balance sheet financial instruments:
Off-balance sheet financial instrument amount	23	21
Estimated Fair Value
Financial assets:
Short-term financial assets	1,175,819	2,650,178
Loans	11,307,808	10,572,261
Financial liabilities:
Deposits	16,969,801	16,053,108
Short-term borrowings	9,131	216,057
Estimated Fair Value | Commitments to extend credit
Off-balance sheet financial instruments:
Off-balance sheet financial instrument amount	21,534	25,113
Estimated Fair Value | Standby letters of credit
Off-balance sheet financial instruments:
Off-balance sheet financial instrument amount	1,533	2,122
Estimated Fair Value | Commercial Letters of credit
Off-balance sheet financial instruments:
Off-balance sheet financial instrument amount	23	21
Estimated Fair Value | Level 1
Financial assets:
Short-term financial assets	371,622	300,096
Financial liabilities:
Deposits	12,960,384	12,251,923
Estimated Fair Value | Level 2
Financial assets:
Short-term financial assets	804,197	2,350,082
Financial liabilities:
Deposits	4,009,417	3,801,185
Short-term borrowings	9,131	216,057
Estimated Fair Value | Level 3
Financial assets:
Loans	11,307,808	10,572,261
Estimated Fair Value | Level 3 | Commitments to extend credit
Off-balance sheet financial instruments:
Off-balance sheet financial instrument amount	21,534	25,113
Estimated Fair Value | Level 3 | Standby letters of credit
Off-balance sheet financial instruments:
Off-balance sheet financial instrument amount	1,533	2,122
Estimated Fair Value | Level 3 | Commercial Letters of credit
Off-balance sheet financial instruments:
Off-balance sheet financial instrument amount	$ 23	$ 21