Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	$ 1,156,067	$ 1,158,643
Net LHFI	7,932,212	7,779,948
Acquired loans	208,236	260,850
Deposits	10,104,472	10,056,012
Long-term FHLB advances	250,994	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	380,090	328,206
Securities held to maturity	1,156,067	1,158,643
Deposits	10,104,472	10,056,012
Short-term liabilities	1,389,025	1,309,595
Long-term FHLB advances	250,994	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	380,090	328,206
Securities held to maturity	1,154,415	1,157,046
Deposits	10,105,262	10,059,794
Short-term liabilities	1,389,025	1,309,595
Long-term FHLB advances	250,994	251,050
Junior subordinated debt securities	43,299	41,057
Level 3 [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	7,932,212	7,779,948
Acquired loans	208,236	260,850
Level 3 [Member] | Estimate Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	8,001,968	7,825,009
Acquired loans	$ 208,236	$ 260,850