Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

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, 2018		Year Ended December 31, 2017		Year Ended December 31, 2016
	(Thousands of dollars)
Aircraft	$	35,740	$	39,815	$	44,130
Other	11,718		9,504		9,451
Total	$	47,458	$	49,319	$	53,581

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)
2019	$	29,705	$	6,765	$	36,470
2020	27,406		5,403		32,809
2021	27,272		3,855		31,127
2022	26,758		2,018		28,776
2023	19,170		516		19,686
Thereafter	9,907		761		10,668
	$	140,218	$	19,318	$	159,536