Fair Value Measurements - Assets on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Summary of assets measured at fair value on a recurring basis
Carried at fair value	$ 2,297,123	$ 1,963,546
Loan servicing rights	68,837	63,841	$ 64,604	$ 48,085
Fair Value, Measurements, Recurring [Member]
Summary of assets measured at fair value on a recurring basis
Carried at fair value	2,297,123	1,963,546
Loans held-for-sale	31,636	52,133
Loan servicing rights	68,837	63,841
Total derivatives	28,998	18,681
Total assets measured at fair value on a recurring basis	2,426,594	2,098,201
Total derivatives	13,960	12,357
Total liabilities at fair value	13,960	12,357
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
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
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	2,297,123	1,963,546
Loans held-for-sale	31,636	52,133
Loan servicing rights	0	0
Total derivatives	28,998	18,681
Total assets measured at fair value on a recurring basis	2,357,757	2,034,360
Total derivatives	13,960	12,357
Total liabilities at fair value	13,960	12,357
Fair Value, Measurements, Recurring [Member] | 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	68,837	63,841
Total derivatives	0	0
Total assets measured at fair value on a recurring basis	68,837	63,841
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	11,619	9,376
Total derivatives	12,055	10,139
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	11,619	9,376
Total derivatives	12,055	10,139
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] | Forward contracts related to mortgage loans to be delivered for sale
Summary of assets measured at fair value on a recurring basis
Total derivatives	404	34
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	404	34
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	1,774	1,222
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	1,774	1,222
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	1,501	2,184
Total derivatives	1,501	2,184
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	1,501	2,184
Total derivatives	1,501	2,184
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	14,104	5,899
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	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	14,104	5,899
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	0
Fair Value, Measurements, Recurring [Member] | Government sponsored agencies
Summary of assets measured at fair value on a recurring basis
Carried at fair value	193,018	202,916
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
Carried 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
Carried at fair value	193,018	202,916
Fair Value, Measurements, Recurring [Member] | Government sponsored agencies | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Carried 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
Carried at fair value	410,401	345,970
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
Carried 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
Carried at fair value	410,401	345,970
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
Carried 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
Carried at fair value	142,734	150,131
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
Carried 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
Carried at fair value	142,734	150,131
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
Carried at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Collateralized mortgage obligations
Summary of assets measured at fair value on a recurring basis
Carried at fair value	1,277,976	1,033,845
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
Carried 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
Carried at fair value	1,277,976	1,033,845
Fair Value, Measurements, Recurring [Member] | 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
Fair Value, Measurements, Recurring [Member] | Corporate bonds
Summary of assets measured at fair value on a recurring basis
Carried at fair value	234,916	192,794
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
Carried 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
Carried at fair value	234,916	192,794
Fair Value, Measurements, Recurring [Member] | Corporate bonds | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Trust preferred securities
Summary of assets measured at fair value on a recurring basis
Carried at fair value	36,271	36,066
Fair Value, Measurements, Recurring [Member] | 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
Fair Value, Measurements, Recurring [Member] | Trust preferred securities | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	36,271	36,066
Fair Value, Measurements, Recurring [Member] | 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
Fair Value, Measurements, Recurring [Member] | Preferred stock
Summary of assets measured at fair value on a recurring basis
Carried at fair value	1,807	1,824
Fair Value, Measurements, Recurring [Member] | Preferred stock | 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
Fair Value, Measurements, Recurring [Member] | Preferred stock | Significant Other Observable Inputs (Level 2)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	1,807	1,824
Fair Value, Measurements, Recurring [Member] | Preferred stock | Significant Unobservable Inputs (Level 3)
Summary of assets measured at fair value on a recurring basis
Carried at fair value	$ 0	$ 0