Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands		6 Months Ended		12 Months Ended
		Jun. 30, 2018		Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Abstract]
Valuation allowance		$ 2,328		$ 2,198
Recurring
Assets, Fair Value Disclosure [Abstract]
U.S. Treasury securities		30,780		30,445
U.S. government agency securities		173,229		180,801
Mortgage-backed securities		1,239,031		1,263,819
State and municipal securities		1,207,181		784,612
Agency-backed securities		240,446		173,292
Corporate notes and other		69,461		82,314
Total investment securities available-for-sale		2,960,128		2,515,283
Other Investments		47,983		28,874
Other assets		27,494		11,812
Total assets at fair value		3,035,605		2,555,969
Liabilities at fair value: [Abstract]
Other liabilities		23,712		13,886
Total liabilities at fair value		23,712		13,886
Recurring | Quoted market prices in an active market (Level 1)
Assets, Fair Value Disclosure [Abstract]
U.S. Treasury securities		0		0
U.S. government agency securities		0		0
Mortgage-backed securities		0		0
State and municipal securities		0		0
Agency-backed securities		0		0
Corporate notes and other		0		0
Total investment securities available-for-sale		0		0
Other Investments		0		0
Other assets		0		0
Total assets at fair value		0		0
Liabilities at fair value: [Abstract]
Other liabilities		0		0
Total liabilities at fair value		0		0
Recurring | Models with significant observable market parameters (Level 2)
Assets, Fair Value Disclosure [Abstract]
U.S. Treasury securities		30,780		30,445
U.S. government agency securities		173,229		180,801
Mortgage-backed securities		1,239,031		1,263,819
State and municipal securities		1,191,891		767,583
Agency-backed securities		240,446		173,292
Corporate notes and other		69,461		82,314
Total investment securities available-for-sale		2,944,838		2,498,254
Other Investments		24,405		0
Other assets		27,494		11,812
Total assets at fair value		2,996,737		2,510,066
Liabilities at fair value: [Abstract]
Other liabilities		23,712		13,886
Total liabilities at fair value		23,712		13,886
Recurring | Models with significant unobservable market parameters (Level 3)
Assets, Fair Value Disclosure [Abstract]
U.S. Treasury securities		0		0
U.S. government agency securities		0		0
Mortgage-backed securities		0		0
State and municipal securities		15,290		17,029
Agency-backed securities		0		0
Corporate notes and other		0		0
Total investment securities available-for-sale		15,290		17,029
Other Investments		23,578		28,874
Other assets		0		0
Total assets at fair value		38,868		45,903
Liabilities at fair value: [Abstract]
Other liabilities		0		0
Total liabilities at fair value		0		0
Nonrecurring
Assets, Fair Value Disclosure [Abstract]
Total assets at fair value		55,333		50,554
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Abstract]
Total losses on impaired loans, net		(35)	[1]	(4)
Gain loss on Other real estate owned		15		203
Nonaccrual loans, net	[1]	35,548		22,723
Other real estate owned		19,785		27,831
Total gains (losses) for the year-to-date period then ended		(20)		199
Nonrecurring | Quoted market prices in an active market (Level 1)
Assets, Fair Value Disclosure [Abstract]
Total assets at fair value		0		0
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Abstract]
Nonaccrual loans, net	[1]	0		0
Other real estate owned		0		0
Nonrecurring | Models with significant observable market parameters (Level 2)
Assets, Fair Value Disclosure [Abstract]
Total assets at fair value		0		0
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Abstract]
Nonaccrual loans, net	[1]	0		0
Other real estate owned		0		0
Nonrecurring | Models with significant unobservable market parameters (Level 3)
Assets, Fair Value Disclosure [Abstract]
Total assets at fair value		55,333		50,554
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Abstract]
Nonaccrual loans, net	[1]	35,548		22,723
Other real estate owned		$ 19,785		$ 27,831

[1] Amount is net of valuation allowance of $2.3 million and $2.2 million at June 30, 2018 and December 31, 2017, respectively, as required by ASC 310-10, "Receivables."