Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	$ 1,056,486	$ 1,158,643
Net LHFI	8,493,234	7,779,948
Acquired loans	257,438	260,850
Deposits	10,577,512	10,056,012
Long-term FHLB advances	946	251,049
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	336,383	328,206
Securities held to maturity	1,056,486	1,158,643
Deposits	10,577,512	10,056,012
Short-term liabilities	1,440,876	1,309,595
Long-term FHLB advances	946	251,049
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	336,383	328,206
Securities held to maturity	1,046,247	1,157,046
Deposits	10,577,858	10,059,794
Short-term liabilities	1,440,876	1,309,595
Long-term FHLB advances	946	251,050
Junior subordinated debt securities	45,773	41,057
Level 3 [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	8,493,234	7,779,948
Acquired loans	257,438	260,850
Level 3 [Member] | Estimate Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	8,507,469	7,825,009
Acquired loans	$ 257,438	$ 260,850