Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies Disclosure [Abstract]

Summary of Rental Expense Incurred under Leases

Rental expense incurred under these leases consisted of the following:

	Year Ended December 31, 2017		Year Ended December 31, 2016		Year Ended December 31, 2015
	(Thousands of dollars)
Aircraft	$	39,815	$	44,130	$	45,360
Other	9,504		9,451		8,358
Total	$	49,319	$	53,581	$	53,718

Summary of Remaining Aggregate Lease Commitments Under Operating Leases Having Initial Non-Cancelable Terms in Excess of One Year

The table includes renewal periods on the principal operating facility lease.

	Aircraft		Other		Total
	(Thousands of dollars)
2018	$	35,723	$	5,157	$	40,880
2019	31,245		3,670		34,915
2020	27,406		3,308		30,714
2021	27,272		2,468		29,740
2022	26,758		1,153		27,911
Thereafter	29,077		9,832		38,909
	$	177,481	$	25,588	$	203,069