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Uncertain Tax Positions (Tables)
 12 Months Ended
Dec. 31, 2017
Income Tax Uncertainties [Abstract]  
Summary of Income Tax Contingencies
The table below presents a reconciliation of the beginning and ending balances of the liability for unrecognized tax benefits, excluding interest and penalties, for the years ended December 31, 2017, 2016, and 2015 (in thousands of dollars):
 
2017
 
2016
 
2015
Balance at January 1
$
19,478

 
$
22,042

 
$
22,798

Increases in unrecognized tax benefits related to the current year
2,512

 
1,700

 
2,917

Increases in unrecognized tax benefits related to prior years
2,430

 
29

 
2,276

Decreases in unrecognized tax benefits related to prior years
(793
)
 
—

 
(1,973
)
Decreases in unrecognized tax benefits related to settlements
(2,075
)
 
—

 
(437
)
Decreases in unrecognized tax benefits due to the lapse of the applicable statute of limitations
(8,378
)
 
(4,293
)
 
(3,539
)
Balance at December 31
$
13,174

 
$
19,478

 
$
22,042