Fair Value - Financial Instruments not Required to be Carried at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Lease and lease commitments excluded
Financing leases	$ 161,800	$ 165,100
Capital lease obligations	34
Book Value
Financial assets:
Cash and cash equivalents	943,800	1,034,644
Loans held for sale	2,037	556
Loans	12,475,920	12,112,303
Financial liabilities:
Deposits	17,395,538	17,612,122
Long-term debt	200,000
Estimated Fair Value
Financial assets:
Cash and cash equivalents	943,791	1,034,644
Loans held for sale	2,044	559
Loans	12,364,858	12,426,506
Financial liabilities:
Deposits	17,372,044	17,598,633
Long-term debt	200,903
Estimated Fair Value | Level 1
Financial assets:
Cash and cash equivalents	332,102	367,084
Financial liabilities:
Deposits	13,506,195	13,438,240
Estimated Fair Value | Level 2
Financial assets:
Cash and cash equivalents	611,689	667,560
Loans held for sale	2,044	559
Financial liabilities:
Deposits	3,865,849	4,160,393
Long-term debt	200,903
Estimated Fair Value | Level 3
Financial assets:
Loans	$ 12,364,858	$ 12,426,506