Supplemental Balance Sheet Information - Plant, Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Cost	$ 926,956	$ 939,810
Accumulated Depreciation	(732,622)	(734,996)
Net Carrying Amount	194,334	204,814
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Cost	4,835	4,905
Accumulated Depreciation	(4,202)	(4,231)
Net Carrying Amount	633	674
Office Equipment
Property Plant And Equipment [Line Items]
Cost	80,284	80,384
Accumulated Depreciation	(58,943)	(57,295)
Net Carrying Amount	21,341	23,089
Transportation and Service Equipment
Property Plant And Equipment [Line Items]
Cost	15,653	16,140
Accumulated Depreciation	(4,576)	(4,291)
Net Carrying Amount	11,077	11,849
Major Equipment
Property Plant And Equipment [Line Items]
Cost	783,155	794,153
Accumulated Depreciation	(635,361)	(639,452)
Net Carrying Amount	147,794	154,701
Tenant Improvements
Property Plant And Equipment [Line Items]
Cost	42,689	43,888
Accumulated Depreciation	(29,467)	(29,665)
Net Carrying Amount	13,222	14,223
Buildings and Land
Property Plant And Equipment [Line Items]
Cost	340	340
Accumulated Depreciation	(73)	(62)
Net Carrying Amount	$ 267	$ 278