Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	$ 1,023,975	$ 1,056,486
Net LHFI	8,432,750	8,493,234
Acquired loans	211,182	257,438
Deposits	10,975,801	10,577,512
Long-term FHLB advances	929	946
Junior subordinated debt securities	61,856	61,856
Level 2 [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	315,388	336,383
Securities held to maturity	1,023,975	1,056,486
Deposits	10,975,801	10,577,512
Short-term liabilities	717,522	1,440,876
Long-term FHLB advances	929	946
Junior subordinated debt securities	61,856	61,856
Level 2 [Member] | Estimate Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	315,388	336,383
Securities held to maturity	998,043	1,046,247
Deposits	10,974,198	10,577,858
Short-term liabilities	717,522	1,440,876
Long-term FHLB advances	853	946
Junior subordinated debt securities	49,485	45,773
Level 3 [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	8,432,750	8,493,234
Acquired loans	211,182	257,438
Level 3 [Member] | Estimate Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	8,255,486	8,507,469
Acquired loans	$ 211,182	$ 257,438