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, 2015	Dec. 31, 2014	Dec. 31, 2013
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	44	46	181
Weighted average grant date fair value (per share)	$ 37.11	$ 31.86	$ 29.44
Aggregate grant date fair value	$ 1,613	$ 1,463	$ 5,325
Compensation expense	3,889	5,239	4,419
Estimated tax benefit (charge)	709	$ 1,233	$ 1,047
Total unrecognized stock based compensation expense	$ 300
Period for recognition of unrecognized expense	1 year
Award activity:
Unvested at beginning of period, shares	433
Granted, shares	44	46	181
Vested, shares	(202)
Unvested at end of period, shares	275	433
Unvested at beginning of period, weighted average grant date fair value	$ 36.41
Granted, weighted average grant date fair value	37.11
Vested, weighted average grant date fair value	29.08
Unvested at end of period, weighted average grant date fair value	$ 41.90	$ 36.41
Restricted stock units
Summary of awards:
Granted, shares	1,534	1,867	1,803
Aggregate grant date fair value	$ 55,540	$ 57,134	$ 51,829
Compensation expense	48,937	41,879	32,874
Estimated tax benefit (charge)	13,271	$ 11,607	$ 8,813
Total unrecognized stock based compensation expense	$ 61,000
Period for recognition of unrecognized expense	1 year 3 months 18 days
Vesting period required	3 years
Award activity:
Unvested at beginning of period, shares	3,248
Granted, shares	1,534	1,867	1,803
Vested, shares	(1,536)
Forfeited, shares	(176)
Unvested at end of period, shares	3,070	3,248
Unvested at beginning of period, weighted average grant date fair value	$ 28.93
Granted, weighted average grant date fair value	36.20
Vested, weighted average grant date fair value	27.67
Forfeited, weighted average grant date fair value	32.45
Unvested at end of period, weighted average grant date fair value	$ 32.99	$ 28.93
Performance units
Summary of awards:
Granted, shares	781	572	656
Potential maximum share payout	1,563	1,144	1,312
Aggregate grant date fair value	$ 28,082	$ 16,345	$ 17,753
Compensation expense	12,942	24,089	1,041
Estimated tax benefit (charge)	1,459	$ 4,250	$ (176)
Total unrecognized stock based compensation expense	$ 27,900
Period for recognition of unrecognized expense	1 year 10 months 24 days
Vesting period required	3 years
Award activity:
Unvested at beginning of period, shares	2,540
Granted, shares	781	572	656
Vested, shares	(750)
Forfeited, shares	(63)
Performance driven reduction, shares	(627)
Unvested at end of period, shares	1,881	2,540
Unvested at beginning of period, weighted average grant date fair value	$ 25.24
Granted, weighted average grant date fair value	35.94
Vested, weighted average grant date fair value	19.35
Forfeited, weighted average grant date fair value	30.03
Performance driven reduction, weighted average grant date fair value	27.74
Unvested at end of period, weighted average grant date fair value	$ 31.02	$ 25.24
Performance units | Minimum
Summary of awards:
Final payout percentage	0.00%
Performance units | Maximum
Summary of awards:
Final payout percentage	200.00%