Lease Accounting (Major Categories of Operating Lease Portfolio) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Operating Leased Assets [Line Items]
Total Right of Use Assets, net	$ 200,788	$ 208,929	$ 0
Accrued liabilities, current	31,560	33,016
Other long-term liabilities, non-current	163,649	171,348
Total Lease Liability	195,209	204,364
Transportation Equipment
Operating Leased Assets [Line Items]
Total Right of Use Assets, net	113,384	117,727
Total Lease Liability	116,889
Office Space & Equipment
Operating Leased Assets [Line Items]
Total Right of Use Assets, net	13,316	14,194
Total Lease Liability	13,354
Facilities and Equipment
Operating Leased Assets [Line Items]
Total Right of Use Assets, net	74,088	$ 77,008
Total Lease Liability	$ 64,966