Employee Benefits - Multiemployer Plans (Details) - Multiemployer Plans, Pension - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
International Union of Operating Engineers - Employers Construction Industry Retirement Plan - Local 302 and 612 Trust Funds
Multiemployer Plans [Line Items]
Certified Zone Status	Green	Green
Contributions	$ 2,482	$ 1,974	$ 2,158
Associated General Contractors of Washington Carpenter, Piledrivers, and Millwrights
Multiemployer Plans [Line Items]
Certified Zone Status	Green	Green
Contributions	$ 932	$ 693	938
Alaska Carpenters Trust Fund
Multiemployer Plans [Line Items]
Certified Zone Status	Green	Green
Contributions	$ 328	$ 396	889
Alaska Laborers Trust Fund
Multiemployer Plans [Line Items]
Certified Zone Status	Yellow	Yellow
Contributions	$ 321	$ 218	$ 126