Fair Value Measurements - Assets and Liabilities on a Recurring Basis (Details) - USD ($)	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Summary of assets measured at fair value on a recurring basis
Carried at fair value	$ 3,369,872,000		$ 3,021,832,000
Loan servicing rights	60,658,000	$ 64,701,000	71,013,000	$ 70,364,000	$ 68,837,000	$ 63,841,000
Transfers between levels	0
Measured on a Recurring Basis
Summary of assets measured at fair value on a recurring basis
Carried at fair value	3,369,872,000		3,021,832,000
Loans held-for-sale	33,019,000		85,030,000
Loan servicing rights	60,658,000		71,013,000
Total derivatives	73,068,000		38,595,000
Total assets measured at fair value on a recurring basis	3,536,617,000		3,216,470,000
Total derivatives	97,179,000		32,379,000
Total liabilities at fair value	97,179,000		32,379,000
Measured on a Recurring Basis | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	0		0
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	0		0
Total derivatives	0		0
Total liabilities at fair value	0		0
Measured on a Recurring Basis | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	3,369,872,000		3,021,832,000
Loans held-for-sale	33,019,000		85,030,000
Loan servicing rights	0		0
Total derivatives	73,068,000		38,595,000
Total assets measured at fair value on a recurring basis	3,475,959,000		3,145,457,000
Total derivatives	97,179,000		32,379,000
Total liabilities at fair value	97,179,000		32,379,000
Measured on a Recurring Basis | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	0		0
Loans held-for-sale	0		0
Loan servicing rights	60,658,000		71,013,000
Total derivatives	0		0
Total assets measured at fair value on a recurring basis	60,658,000		71,013,000
Total derivatives	0		0
Total liabilities at fair value	0		0
Measured on a Recurring Basis | Customer-initiated derivatives
Summary of assets measured at fair value on a recurring basis
Total derivatives	69,024,000		26,680,000
Total derivatives	70,707,000		27,664,000
Measured on a Recurring Basis | 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
Measured on a Recurring Basis | Customer-initiated derivatives | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Total derivatives	69,024,000		26,680,000
Total derivatives	70,707,000		27,664,000
Measured on a Recurring Basis | 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
Measured on a Recurring Basis | Forward contracts related to mortgage loans to be delivered for sale
Summary of assets measured at fair value on a recurring basis
Total derivatives	661,000		719,000
Measured on a Recurring Basis | 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
Measured on a Recurring Basis | 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	661,000		719,000
Measured on a Recurring Basis | 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
Measured on a Recurring Basis | Interest rate lock commitments
Summary of assets measured at fair value on a recurring basis
Total derivatives	2,687,000		1,049,000
Measured on a Recurring Basis | 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
Measured on a Recurring Basis | Interest rate lock commitments | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Total derivatives	2,687,000		1,049,000
Measured on a Recurring Basis | Interest rate lock commitments | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Total derivatives	0		0
Measured on a Recurring Basis | Power Equity CD
Summary of assets measured at fair value on a recurring basis
Total derivatives	751,000		718,000
Total derivatives	751,000		718,000
Measured on a Recurring Basis | 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
Measured on a Recurring Basis | Power Equity CD | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Total derivatives	751,000		718,000
Total derivatives	751,000		718,000
Measured on a Recurring Basis | 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
Measured on a Recurring Basis | Risk management derivatives
Summary of assets measured at fair value on a recurring basis
Total derivatives	606,000		10,148,000
Total derivatives	25,060,000		3,278,000
Measured on a Recurring Basis | 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		0
Total derivatives	0		0
Measured on a Recurring Basis | Risk management derivatives | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Total derivatives	606,000		10,148,000
Total derivatives	25,060,000		3,278,000
Measured on a Recurring Basis | Risk management 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
Measured on a Recurring Basis | Government and government-sponsored enterprises
Summary of assets measured at fair value on a recurring basis
Carried at fair value	295,726,000		351,700,000
Measured on a Recurring Basis | Government and government-sponsored enterprises | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	0		0
Measured on a Recurring Basis | Government and government-sponsored enterprises | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	295,726,000		351,700,000
Measured on a Recurring Basis | Government and government-sponsored enterprises | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	0		0
Measured on a Recurring Basis | State and political subdivisions
Summary of assets measured at fair value on a recurring basis
Carried at fair value	444,085,000		516,286,000
Measured on a Recurring Basis | 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
Carried at fair value	0		0
Measured on a Recurring Basis | State and political subdivisions | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	444,085,000		516,286,000
Measured on a Recurring Basis | State and political subdivisions | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	0		0
Measured on a Recurring Basis | Residential mortgage-backed securities
Summary of assets measured at fair value on a recurring basis
Carried at fair value	353,622,000		213,428,000
Measured on a Recurring Basis | 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
Carried at fair value	0		0
Measured on a Recurring Basis | Residential mortgage-backed securities | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	353,622,000		213,428,000
Measured on a Recurring Basis | Residential mortgage-backed securities | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	0		0
Measured on a Recurring Basis | Collateralized mortgage obligations
Summary of assets measured at fair value on a recurring basis
Carried at fair value	1,939,695,000		1,601,298,000
Measured on a Recurring Basis | Collateralized mortgage obligations | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	0		0
Measured on a Recurring Basis | Collateralized mortgage obligations | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	1,939,695,000		1,601,298,000
Measured on a Recurring Basis | Collateralized mortgage obligations | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	0		0
Measured on a Recurring Basis | Corporate bonds
Summary of assets measured at fair value on a recurring basis
Carried at fair value	289,312,000		293,063,000
Measured on a Recurring Basis | Corporate bonds | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	0		0
Measured on a Recurring Basis | Corporate bonds | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	289,312,000		293,063,000
Measured on a Recurring Basis | Corporate bonds | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	0		0
Measured on a Recurring Basis | Trust preferred securities
Summary of assets measured at fair value on a recurring basis
Carried at fair value	47,432,000		46,057,000
Measured on a Recurring Basis | Trust preferred securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	0		0
Measured on a Recurring Basis | Trust preferred securities | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	47,432,000		46,057,000
Measured on a Recurring Basis | Trust preferred securities | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	$ 0		$ 0