Summary of Significant Accounting Policies - Life Insurance Policies (Details)	3 Months Ended				12 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) Individual	Apr. 01, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) Individual	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Supplementary Information For Corporate Owned Life Insurance [Line Items]
Number of former employees covered under insurance policies | Individual		20			20
Average age of former employees covered under insurance policies					74 years
Gross cash surrender value of life insurance		$ 31,547,000			$ 31,547,000	$ 30,143,000
Loans on cash surrender value		0			0	0
Gross death benefit of life insurance contracts		42,200,000			42,200,000	41,400,000
Life insurance proceeds to be received upon the death of participants		41,600,000			41,600,000
Life insurance gain to be recognized upon death of participants		10,100,000			10,100,000
Non-taxable life insurance gain		$ 400,000		$ 200,000	$ 390,000		$ 246,000
Life insurance proceeds			$ 400,000			$ 394,000
Scenario Forecast [Member]
Supplementary Information For Corporate Owned Life Insurance [Line Items]
Life insurance proceeds	$ 1,100,000