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, 2016	Dec. 31, 2015	Dec. 31, 2014
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	48	44	46
Weighted average grant date fair value (per share)	$ 34.05	$ 37.11	$ 31.86
Aggregate grant date fair value	$ 1,650	$ 1,613	$ 1,463
Compensation expense	2,078	3,889	5,239
Estimated tax benefit (charge)	129	$ 709	$ 1,233
Total unrecognized stock based compensation expense	$ 0
Award activity:
Unvested at beginning of period, shares	275
Granted, shares	48	44	46
Vested, shares	(132)
Unvested at end of period, shares	191	275
Unvested at beginning of period, weighted average grant date fair value	$ 41.90
Granted, weighted average grant date fair value	34.05
Vested, weighted average grant date fair value	36.35
Unvested at end of period, weighted average grant date fair value	$ 43.72	$ 41.90
Restricted stock units
Summary of awards:
Granted, shares	1,117	1,534	1,867
Aggregate grant date fair value	$ 38,681	$ 55,540	$ 57,134
Compensation expense	46,209	48,937	41,879
Estimated tax benefit (charge)	12,701	$ 13,271	$ 11,607
Total unrecognized stock based compensation expense	$ 49,900
Period for recognition of unrecognized expense	1 year 4 months 24 days
Vesting period required	3 years
Award activity:
Unvested at beginning of period, shares	3,070
Granted, shares	1,117	1,534	1,867
Vested, shares	(1,559)
Forfeited, shares	(137)
Unvested at end of period, shares	2,491	3,070
Unvested at beginning of period, weighted average grant date fair value	$ 32.99
Granted, weighted average grant date fair value	34.64
Vested, weighted average grant date fair value	31.83
Forfeited, weighted average grant date fair value	36.37
Unvested at end of period, weighted average grant date fair value	$ 34.30	$ 32.99
Performance units
Summary of awards:
Granted, shares	738	781	572
Potential maximum share payout	1,477	1,563	1,144
Aggregate grant date fair value	$ 25,578	$ 28,082	$ 16,345
Compensation expense	21,647	12,942	24,089
Estimated tax benefit (charge)	3,152	$ 1,459	$ 4,250
Total unrecognized stock based compensation expense	$ 29,800
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	1,881
Granted, shares	738	781	572
Vested, shares	(746)
Forfeited, shares	(28)
Performance driven reduction, shares	(477)
Unvested at end of period, shares	1,368	1,881
Unvested at beginning of period, weighted average grant date fair value	$ 31.02
Granted, weighted average grant date fair value	34.64
Vested, weighted average grant date fair value	27.00
Forfeited, weighted average grant date fair value	34.12
Performance driven reduction, weighted average grant date fair value	34.60
Unvested at end of period, weighted average grant date fair value	$ 32.83	$ 31.02
Performance units | Minimum
Summary of awards:
Final payout percentage	0.00%
Performance units | Maximum
Summary of awards:
Final payout percentage	200.00%