Loans and Allowance for Loan Losses - Changes in the carrying value of acquired credit impaired loans (Details)	3 Months Ended	6 Months Ended
	Dec. 31, 2015 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Percentage of loss adjustment based on most current collateral value	90
Maximum
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Threshold limit of loans for risk assessment by loan officers	$ 500,000
Minimum
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Threshold limit of loans for risk assessment by loan officers	$ 100,000
Acquired credit impaired loans
Changes in the carrying value of acquired loans at the acquisition date
Balance at the beginning of the period		$ 485,119,000	$ 618,803,000
Net reductions for payments, foreclosures, and accretion		(65,139,000)	(67,025,000)
Change in the allowance for loan losses on acquired loans		(19,000)	201,000
Balance at the end of the period		419,961,000	551,979,000
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Balance at beginning of period		116,754,000	133,096,000
PSC acquisition Day 1 adjustment			(1,460,000)
Contractual interest income		(14,189,000)	(17,050,000)
Accretion on acquired credit impaired loans		(10,287,000)	(9,013,000)
Reclass of nonaccretable difference due to improvement in expected cash flows		8,468,000	16,453,000
Other changes, net		(133,000)	(222,000)
Balance at end of period		100,613,000	121,804,000
Decline in accretable yield balance		16,100,000
Accretion income		24,500,000
Improved expected cash flows		8,500,000
Contractual principal and interest		538,730,000	717,180,000	$ 626,691,000
Acquired non-credit impaired loans accounted under FASB 310 20
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Contractual principal and interest		2,200,000,000	3,100,000,000
After fair value adjustment		2,200,000,000	3,100,000,000
Fair value adjustment		$ 26,900,000	$ 43,600,000