Fair Value Measurements - Assets on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	$ 2,029,672		$ 1,234,964
Loan servicing rights	62,195	$ 64,522	48,085	$ 41,928	$ 0	$ 0
U.S. Treasury securities
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	5,799		5,793
Government sponsored agencies
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	277,848		215,011
State and political subdivisions
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	352,619		300,088
Residential mortgage-backed securities
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	297,319		272,282
Collateralized mortgage obligations
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	879,429		320,025
Corporate bonds
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	178,741		89,474
Preferred stock and trust preferred securities
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	37,917		32,291
Fair Value, Measurements, Recurring [Member]
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	2,029,672		1,234,964
Loans held-for-sale	87,198		81,830
Loan servicing rights	62,195		48,085
Total derivatives	14,713		8,215
Total assets measured at fair value on a recurring basis	2,193,778		1,373,094
Total derivatives	11,739		6,359
Total liabilities at fair value	11,739		6,359
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	5,799		5,793
Loans held-for-sale	0		0
Loan servicing rights	0		0
Total derivatives	0		0
Total assets measured at fair value on a recurring basis	5,799		5,793
Total derivatives	0		0
Total liabilities at fair value	0		0
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	2,023,873		1,229,171
Loans held-for-sale	87,198		81,830
Loan servicing rights	0		0
Total derivatives	14,713		8,215
Total assets measured at fair value on a recurring basis	2,125,784		1,319,216
Total derivatives	11,739		6,359
Total liabilities at fair value	11,739		6,359
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Loans held-for-sale	0		0
Loan servicing rights	62,195		48,085
Total derivatives	0		0
Total assets measured at fair value on a recurring basis	62,195		48,085
Total derivatives	0		0
Total liabilities at fair value	0		0
Fair Value, Measurements, Recurring [Member] | Customer-initiated derivatives
Summary of assets measured at fair value on a recurring basis
Total derivatives	8,719		4,406
Total derivatives	9,274		4,141
Fair Value, Measurements, Recurring [Member] | Customer-initiated derivatives | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0		0
Total derivatives	0		0
Fair Value, Measurements, Recurring [Member] | Customer-initiated derivatives | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Total derivatives	8,719		4,406
Total derivatives	9,274		4,141
Fair Value, Measurements, Recurring [Member] | Customer-initiated derivatives | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0		0
Total derivatives	0		0
Fair Value, Measurements, Recurring [Member] | Foreign exchange forwards
Summary of assets measured at fair value on a recurring basis
Total derivatives	60
Total derivatives	55
Fair Value, Measurements, Recurring [Member] | Foreign exchange forwards | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0
Total derivatives	0
Fair Value, Measurements, Recurring [Member] | Foreign exchange forwards | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Total derivatives	60
Total derivatives	55
Fair Value, Measurements, Recurring [Member] | Foreign exchange forwards | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0
Total derivatives	0
Fair Value, Measurements, Recurring [Member] | Forward contracts related to mortgage loans to be delivered for sale
Summary of assets measured at fair value on a recurring basis
Total derivatives	660		635
Fair Value, Measurements, Recurring [Member] | Forward contracts related to mortgage loans to be delivered for sale | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0		0
Fair Value, Measurements, Recurring [Member] | Forward contracts related to mortgage loans to be delivered for sale | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Total derivatives	660		635
Fair Value, Measurements, Recurring [Member] | Forward contracts related to mortgage loans to be delivered for sale | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0		0
Fair Value, Measurements, Recurring [Member] | Interest rate lock commitments
Summary of assets measured at fair value on a recurring basis
Total derivatives	1,799		956
Fair Value, Measurements, Recurring [Member] | Interest rate lock commitments | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0		0
Fair Value, Measurements, Recurring [Member] | Interest rate lock commitments | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Total derivatives	1,799		956
Fair Value, Measurements, Recurring [Member] | Interest rate lock commitments | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0		0
Fair Value, Measurements, Recurring [Member] | Power Equity CD
Summary of assets measured at fair value on a recurring basis
Total derivatives	2,301		2,218
Total derivatives	2,301		2,218
Fair Value, Measurements, Recurring [Member] | Power Equity CD | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0		0
Total derivatives	0		0
Fair Value, Measurements, Recurring [Member] | Power Equity CD | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Total derivatives	2,301		2,218
Total derivatives	2,301		2,218
Fair Value, Measurements, Recurring [Member] | Power Equity CD | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0		0
Total derivatives	0		0
Fair Value, Measurements, Recurring [Member] | Risk management derivatives
Summary of assets measured at fair value on a recurring basis
Total derivatives	1,174
Total derivatives	109
Fair Value, Measurements, Recurring [Member] | Risk management derivatives | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0
Total derivatives	0
Fair Value, Measurements, Recurring [Member] | Risk management derivatives | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Total derivatives	1,174
Total derivatives	109
Fair Value, Measurements, Recurring [Member] | Risk management derivatives | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0
Total derivatives	0
Fair Value, Measurements, Recurring [Member] | U.S. Treasury securities
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	5,799		5,793
Fair Value, Measurements, Recurring [Member] | U.S. Treasury securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	5,799		5,793
Fair Value, Measurements, Recurring [Member] | U.S. Treasury securities | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | U.S. Treasury securities | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | Government sponsored agencies
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	277,848		215,011
Fair Value, Measurements, Recurring [Member] | Government sponsored agencies | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | Government sponsored agencies | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	277,848		215,011
Fair Value, Measurements, Recurring [Member] | Government sponsored agencies | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | State and political subdivisions
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	352,619		300,088
Fair Value, Measurements, Recurring [Member] | State and political subdivisions | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | State and political subdivisions | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	352,619		300,088
Fair Value, Measurements, Recurring [Member] | State and political subdivisions | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	297,319		272,282
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	297,319		272,282
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | Collateralized mortgage obligations
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	879,429		320,025
Fair Value, Measurements, Recurring [Member] | Collateralized mortgage obligations | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | Collateralized mortgage obligations | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	879,429		320,025
Fair Value, Measurements, Recurring [Member] | Collateralized mortgage obligations | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | Corporate bonds
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	178,741		89,474
Fair Value, Measurements, Recurring [Member] | Corporate bonds | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | Corporate bonds | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	178,741		89,474
Fair Value, Measurements, Recurring [Member] | Corporate bonds | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | Preferred stock and trust preferred securities
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	37,917		32,291
Fair Value, Measurements, Recurring [Member] | Preferred stock and trust preferred securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	0		0
Fair Value, Measurements, Recurring [Member] | Preferred stock and trust preferred securities | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	37,917		32,291
Fair Value, Measurements, Recurring [Member] | Preferred stock and trust preferred securities | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Available-for-sale, at fair value	$ 0		$ 0