Summary of Significant Accounting Policies - Supplemental Cash Flow Information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Accounting Policies [Abstract]
Right of use assets recognized during the period	$ 81,249,000	$ 0
Cash Transactions:
Interest paid	68,403,000	34,909,000
Income taxes paid, net	550,000	425,000
Noncash Transactions:
Loans charged-off to the allowance for loan losses	6,068,000	8,669,000
Loans foreclosed upon and transferred to other real estate owned	624,000	232,000
Loans foreclosed upon and transferred to other assets	$ 87,000	$ 392,000
Available-for-sale securities transferred to Held-to-Maturity			$ 179,800,000