Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	$ 1,168,203	$ 1,187,818
Net LHFI	7,198,354	7,023,766
FDIC indemnification asset	506	738
Deposits	9,633,643	9,588,230
Long-term FHLB advances	501,124	501,155
Subordinated notes	49,977	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	228,498	278,001
Securities held to maturity	1,168,203	1,187,818
Deposits	9,633,643	9,588,230
Short-term liabilities	877,821	853,659
Long-term FHLB advances	501,124	501,155
Subordinated notes	49,977	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	228,498	278,001
Securities held to maturity	1,199,213	1,195,367
Deposits	9,638,119	9,592,531
Short-term liabilities	877,821	853,659
Long-term FHLB advances	507,128	501,160
Subordinated notes	51,279	51,405
Junior subordinated debt securities	43,299	49,021
Level 3 [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	7,198,354	7,023,766
Acquired loans	351,220	378,419
FDIC indemnification asset	506	738
Level 3 [Member] | Estimate Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	7,337,520	7,136,105
Acquired loans	351,220	378,419
FDIC indemnification asset	$ 506	$ 738