Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity (fair value: $1,195,367-2015; $1,182,846-2014)	$ 1,187,818	$ 1,170,685
Net LHFI	7,023,766	6,379,853
FDIC indemnification asset	738	6,997	$ 14,347	$ 21,774
Deposits	9,588,230	9,698,358
Long-term FHLB advances	501,155	1,253
Subordinated notes	49,969	49,936
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	278,001	317,858
Securities held to maturity (fair value: $1,195,367-2015; $1,182,846-2014)	1,187,818	1,170,685
Deposits	9,588,230	9,698,358
Short-term liabilities	853,659	868,620
Long-term FHLB advances	501,155	1,253
Subordinated notes	49,969	49,936
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	278,001	317,858
Securities held to maturity (fair value: $1,195,367-2015; $1,182,846-2014)	1,195,367	1,182,846
Deposits	9,592,531	9,702,864
Short-term liabilities	853,659	868,620
Long-term FHLB advances	501,160	1,263
Subordinated notes	51,405	53,504
Junior subordinated debt securities	49,021	46,392
Level 3 [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	7,023,766	6,379,853
Acquired loans	378,419	537,350
FDIC indemnification asset	738	6,997
Level 3 [Member] | Estimate Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	7,136,105	6,453,618
Acquired loans	378,419	537,350
FDIC indemnification asset	$ 738	$ 6,997