Summary of Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accumulated other comprehensive loss		$ 580	$ (10,893)
Reinvested earnings		257,313	244,224
Operating earnings		56,869	56,595	$ 52,672
Accounting Standards Update 2016-02 | Subsequent Event
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Increase in balance sheet asset for recognition of ROU assets and lease liabilities for operating leases	$ 11,000
Accounting Standards Update 2018-02
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Increase in balance sheet asset for recognition of ROU assets and lease liabilities for operating leases		1,700
Accumulated other comprehensive loss					$ 1,700
Reinvested earnings					$ 1,700
Accounting Standards Update 2017-07
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating earnings		19,900	1,000
Other pension (benefits) costs		$ 19,900	$ 1,000