Employee Benefits - Multiemployer Plans (Details) - Multiemployer Plans, Pension - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 3,021	$ 2,482	$ 1,974
Washington Laborers
Multiemployer Plans [Line Items]
Certified Zone Status	Green	Green
Contributions	$ 30
Carpenters Retirement Plan of Western Washington
Multiemployer Plans [Line Items]
Certified Zone Status	Green	Green
Contributions	$ 695	$ 932	693
Cement Masons & Plasterers Trust Funds
Multiemployer Plans [Line Items]
Certified Zone Status	Green	Green
Contributions	$ 2
Washington-Idaho-Montana Carpenters-Employers Retirement Trust Fund
Multiemployer Plans [Line Items]
Certified Zone Status	Yellow	Yellow
Contributions	$ 36
Engineers - AGC Retirement Trust of the Inland Empire
Multiemployer Plans [Line Items]
Certified Zone Status	Yellow	Yellow
Contributions	$ 20
Alaska Carpenters Trust Fund
Multiemployer Plans [Line Items]
Certified Zone Status	Yellow	Green
Contributions	$ 377	$ 328	396
Alaska Laborers Trust Fund
Multiemployer Plans [Line Items]
Certified Zone Status	Yellow	Yellow
Contributions	$ 552	$ 321	$ 218