Significant Accounting Policies - Additional Information (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) Office Factor Region Fee RevenueCategory Contract	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of offices | Office		196
Percentage of outstanding principal to be repurchased under GNMA optional repurchase program		100.00%
Number of days to pass to be classified as past due LHFI		30 days
Balance of commercial nonaccrual loans identified for impairment analysis, minimum		$ 500,000
Components in allowance for loan losses methodology | Factor		3
Key market regions | Region		5
Finite-lived intangible assets, average useful life		20 years
Securities with limited marketability		$ 32,200,000	$ 67,700,000
Number of types of interchange fees | Fee		2
Other real estate sales, net gains		$ 700,000	$ 2,087,000	$ 7,031,000	[1]
Number of trust management revenue categories | RevenueCategory		5
Time period between service obligation completed and payment received from trust customer		30 days
Number of general categories of insurance contracts | Contract		4
Corporate income tax rate		21.00%	35.00%
Stranded tax effects, reclassify from accumulated other comprehensive loss to retained earnings	$ 8,500,000		$ 8,500,000
Accounting Standard Update 2017-07 [Member]
Summary Of Significant Accounting Policies [Line Items]
Net periodic benefit cost		$ 3,500,000	$ 5,700,000	$ 10,400,000
Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative adjustment		$ 0
Percentage of revenues earned		23.00%
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Summary Of Significant Accounting Policies [Line Items]
Description of variable rate basis for derivative		three-month LIBOR
Period for which cash flow hedges will be used to hedge quarterly interest payments		5 years
Derivative inception date		Dec. 31, 2014
Derivative maturity date		Dec. 31, 2019
Credit Card Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of days past due loans are to be charged-off		180 days
Commercial Credits [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of days past due for loan to be classified as nonaccrual		90 days
Non-Business Purpose Credits [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of days past due for loan to be classified as nonaccrual		120 days
Number of days past due loans are to be charged-off		120 days
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of days mortgage LHFS are retained on balance sheet		30 days
Minimum [Member] | Personal Insurance Contracts [Member]
Summary Of Significant Accounting Policies [Line Items]
Insurance contracts payment received period		30 days
Minimum [Member] | Employee Benefits Insurance Contracts [Member]
Summary Of Significant Accounting Policies [Line Items]
Insurance contracts payment received period		60 days
Minimum [Member] | Furniture and Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of the assets		3 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of days mortgage LHFS are retained on balance sheet		45 days
Maximum [Member] | Personal Insurance Contracts [Member]
Summary Of Significant Accounting Policies [Line Items]
Insurance contracts payment received period		60 days
Maximum [Member] | Employee Benefits Insurance Contracts [Member]
Summary Of Significant Accounting Policies [Line Items]
Insurance contracts payment received period		90 days
Maximum [Member] | Buildings [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of the assets		39 years
Maximum [Member] | Furniture and Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of the assets		10 years
Maximum [Member] | 1-4 Family Residential Real Estate [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of days past due loans are to be charged-off		180 days

[1] The changes and gains, net on other real estate for the year ended December 31, 2016 include covered other real estate.