Share Capital - Restricted stock, restricted stock units and performance units (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Minimum
Summary of awards:
Vesting period required	3 years
Maximum
Summary of awards:
Vesting period required	4 years
Restricted stock awards
Summary of awards:
Granted, shares	36	48	44
Weighted average grant date fair value (per share)	$ 43.18	$ 34.05	$ 37.11
Aggregate grant date fair value	$ 1,575	$ 1,650	$ 1,613
Compensation expense	1,575	2,078	3,889
Estimated tax benefit (charge)	0	$ 129	$ 709
Total unrecognized stock based compensation expense	$ 0
Award activity:
Unvested at beginning of period, shares	191
Granted, shares	36	48	44
Vested, shares	(81)
Unvested at end of period, shares	146	191
Unvested at beginning of period, weighted average grant date fair value	$ 43.72
Granted, weighted average grant date fair value	43.18
Vested, weighted average grant date fair value	53.25
Unvested at end of period, weighted average grant date fair value	$ 38.38	$ 43.72
Restricted stock units
Summary of awards:
Granted, shares	1,074	1,117	1,534
Aggregate grant date fair value	$ 43,506	$ 38,681	$ 55,540
Compensation expense	38,778	46,209	48,937
Estimated tax benefit (charge)	7,016	$ 12,701	$ 13,271
Total unrecognized stock based compensation expense	$ 48,500
Period for recognition of unrecognized expense	1 year 6 months
Vesting period required	3 years
Award activity:
Unvested at beginning of period, shares	2,491
Granted, shares	1,074	1,117	1,534
Vested, shares	(1,310)
Forfeited, shares	(141)
Unvested at end of period, shares	2,114	2,491
Unvested at beginning of period, weighted average grant date fair value	$ 34.30
Granted, weighted average grant date fair value	40.49
Vested, weighted average grant date fair value	33.64
Forfeited, weighted average grant date fair value	36.83
Unvested at end of period, weighted average grant date fair value	$ 37.74	$ 34.30
Performance units
Summary of awards:
Granted, shares	607	738	781
Potential maximum share payout	1,214	1,477	1,563
Aggregate grant date fair value	$ 24,579	$ 25,578	$ 28,082
Compensation expense	11,447	21,647	12,942
Estimated tax benefit (charge)	921	$ 3,152	$ 1,459
Total unrecognized stock based compensation expense	$ 20,100
Period for recognition of unrecognized expense	1 year 9 months 18 days
Vesting period required	3 years
Award activity:
Unvested at beginning of period, shares	1,368
Granted, shares	607	738	781
Vested, shares	(281)
Forfeited, shares	(104)
Performance driven reduction, shares	(707)
Unvested at end of period, shares	883	1,368
Unvested at beginning of period, weighted average grant date fair value	$ 32.83
Granted, weighted average grant date fair value	40.49
Vested, weighted average grant date fair value	28.56
Forfeited, weighted average grant date fair value	37.92
Performance driven reduction, weighted average grant date fair value	34.37
Unvested at end of period, weighted average grant date fair value	$ 37.61	$ 32.83
Performance units | Minimum
Summary of awards:
Final payout percentage	0.00%
Performance units | Maximum
Summary of awards:
Final payout percentage	200.00%