Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands		9 Months Ended		12 Months Ended
		Sep. 30, 2018		Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Abstract]
Valuation allowance		$ 2,378		$ 2,198
Recurring
Assets, Fair Value Disclosure [Abstract]
U.S. Treasury securities		30,528		30,445
U.S. government agency securities		63,185		180,801
Mortgage-backed securities		1,294,545		1,263,819
State and municipal securities		1,212,271		784,612
Agency-backed securities		334,658		173,292
Corporate notes and other		69,395		82,314
Total investment securities available-for-sale		3,004,582		2,515,283
Other Investments		49,351		53,796
Other assets		34,700		11,812
Total assets at fair value		3,088,633		2,580,891
Liabilities at fair value: [Abstract]
Other liabilities		26,727		13,886
Total liabilities at fair value		26,727		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		30,528		30,445
U.S. government agency securities		63,185		180,801
Mortgage-backed securities		1,294,545		1,263,819
State and municipal securities		1,196,917		767,583
Agency-backed securities		334,658		173,292
Corporate notes and other		69,395		82,314
Total investment securities available-for-sale		2,989,228		2,498,254
Other Investments		24,169		24,922
Other assets		34,700		11,812
Total assets at fair value		3,048,097		2,534,988
Liabilities at fair value: [Abstract]
Other liabilities		26,727		13,886
Total liabilities at fair value		26,727		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		15,354		17,029
Agency-backed securities		0		0
Corporate notes and other		0		0
Total investment securities available-for-sale		15,354		17,029
Other Investments		25,182		28,874
Other assets		0		0
Total assets at fair value		40,536		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		51,549		50,554
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Abstract]
Total losses on impaired loans, net		(133)	[1]	(4)
Gain loss on Other real estate owned		259		203
Nonaccrual loans, net	[1]	34,082		22,723
Other real estate owned		17,467		27,831
Total gains (losses) for the year-to-date period then ended		126		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		51,549		50,554
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Abstract]
Nonaccrual loans, net	[1]	34,082		22,723
Other real estate owned		$ 17,467		$ 27,831

[1] Amount is net of valuation allowance of $2.4 million and $2.2 million at September 30, 2018 and December 31, 2017, respectively, as required by ASC 310-10, "Receivables."