Revenue - Revenue Disclosure (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Customer Contract, Lower Range, in Years	1
Customer Contract, Upper Range, in Years	5
SEA-US Contract, Lower Range, in Years		15
SEA-US Contract, Upper Range, in Years		25
Payment terms for customers, lower range	30
Payment term for customers, upper range	180
Indefeasible Right of Use, Lower Range, in Years	15
Indefeasible Right of Use, Upper Range, in Years	25