Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	$ 1,139,754	$ 1,158,643
Net LHFI	8,219,861	7,779,948
Acquired loans	307,487	260,850
Deposits	10,423,391	10,056,012
Long-term FHLB advances	978	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	325,229	328,206
Securities held to maturity	1,139,754	1,158,643
Deposits	10,423,391	10,056,012
Short-term liabilities	1,730,660	1,309,595
Long-term FHLB advances	978	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	325,229	328,206
Securities held to maturity	1,142,285	1,157,046
Deposits	10,419,769	10,059,794
Short-term liabilities	1,730,660	1,309,595
Long-term FHLB advances	978	251,050
Junior subordinated debt securities	44,536	41,057
Level 3 [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	8,219,861	7,779,948
Acquired loans	307,487	260,850
Level 3 [Member] | Estimate Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	8,323,752	7,825,009
Acquired loans	$ 307,487	$ 260,850