Employee Benefits - Multiemployer Plans (Details) - Multiemployer Plans, Pension - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
International Union of Operating Engineers - Employers Construction Industry Retirement Plan - Local 302 and 612 Trust Funds
Multiemployer Plans [Line Items]
Certified Zone Status	[1]	Green	Green
Contributions		$ 1,974	$ 2,158	$ 1,518
Expiration of Collective Bargaining Agreement		Dec. 31, 2018
Associated General Contractors of Washington Carpenter, Piledrivers, and Millwrights
Multiemployer Plans [Line Items]
Certified Zone Status	[1]	Green	Green
Contributions		$ 693	$ 938	748
Expiration of Collective Bargaining Agreement		Dec. 31, 2018
Alaska Carpenters Trust Fund
Multiemployer Plans [Line Items]
Certified Zone Status	[1]	Green	Green
Contributions		$ 396	$ 889	807
Expiration of Collective Bargaining Agreement		Jun. 30, 2020
Alaska Laborers Trust Fund
Multiemployer Plans [Line Items]
Certified Zone Status	[1]	Yellow	Yellow
Contributions		$ 218	$ 126	$ 110
Expiration of Collective Bargaining Agreement		Dec. 31, 2017

[1] The most recent PPA zone status available in 2017 and 2016 is for the plan's year end during 2016 and 2015, respectively. Zone status is based on information received from the plan and is indicative of the plans funding status. Among other factors, plans in the red zone are generally less than 65 percent funded, plans in the orange zone are less than 80 percent funded and have an Accumulated Funding Deficiency in the current year or projected into the next six years, plans in the yellow zone are less than 80 percent funded, and plans in the green zone are at least 80 percent funded.