Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	$ 1,143,234	$ 1,187,818
Net LHFI	7,428,333	7,023,766
FDIC indemnification asset	0	738
Deposits	9,685,701	9,588,230
Long-term FHLB advances	751,075	501,155
Subordinated notes	49,993	49,969
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	384,445	278,001
Securities held to maturity	1,143,234	1,187,818
Deposits	9,685,701	9,588,230
Short-term liabilities	927,710	853,659
Long-term FHLB advances	751,075	501,155
Subordinated notes	49,993	49,969
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	384,445	278,001
Securities held to maturity	1,173,101	1,195,367
Deposits	9,689,282	9,592,531
Short-term liabilities	927,710	853,659
Long-term FHLB advances	751,077	501,160
Subordinated notes	50,484	51,405
Junior subordinated debt securities	40,825	49,021
Level 3 [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	7,428,333	7,023,766
Acquired loans	284,357	378,419
FDIC indemnification asset	0	738
Level 3 [Member] | Estimate Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	7,493,082	7,136,105
Acquired loans	284,357	378,419
FDIC indemnification asset	$ 0	$ 738