Share-Based Compensation and Pensions - Compensation Costs for Share-based Compensation Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation costs	$ 37,999	$ 10,670	$ 26,326
Restricted shares and restricted share units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation costs	6,564	7,641	7,302
Performance share units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation costs	23,582	1,968	5,832
Cash-settled stock appreciation rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation costs	2,575	(3,316)	8,875
Northshore incentive plan | Time-based restricted shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation costs	3,652	2,792	3,156
StarStone incentive plan | Time-based restricted shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation costs	223	0	0
Employee share purchase plan | Employee share purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation costs	411	430	403
Share-based compensation plans | Director | Deferred compensation and ordinary share plan for non-employee directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred compensation and ordinary share plan for non-employee directors	$ 992	$ 1,155	$ 758