Share Capital - Restricted stock, restricted stock units and performance units (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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 (in Shares)	46	181	325
Weighted average grant date fair value (per share)	$ 31.86	$ 29.44	$ 20.92
Aggregate grant date fair value	$ 1,463,000	$ 5,325,000	$ 6,811,000
Compensation expense	5,239,000	4,419,000	5,399,000
Estimated tax benefit (charge)	1,233,000	1,047,000	1,113,000
Total unrecognized stock based compensation expense	2,500,000
Period for recognition of unrecognized expense	1 year
Award activity:
Unvested at beginning of period, shares	593
Granted, shares	46	181	325
Vested, shares	(201)
Forfeited, shares	(5)
Unvested at end of period, shares	433	593
Unvested at beginning of period, weighted average grant date fair value	$ 33.92
Granted, weighted average grant date fair value	$ 31.86
Vested, weighted average grant date fair value	$ 27.79
Forfeited, weighted average grant date fair value	$ 44.01
Unvested at end of period, weighted average grant date fair value	$ 36.41	$ 33.92
Restricted stock units
Summary of awards:
Granted (in Shares)	1,867	1,803	1,312
Aggregate grant date fair value	57,134,000	51,829,000	27,194,000
Compensation expense	41,879,000	32,874,000	24,451,000
Estimated tax benefit (charge)	11,607,000	8,813,000	6,501,000
Total unrecognized stock based compensation expense	60,100,000
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	2,931
Granted, shares	1,867	1,803	1,312
Vested, shares	(1,388)
Forfeited, shares	(162)
Unvested at end of period, shares	3,248	2,931
Unvested at beginning of period, weighted average grant date fair value	$ 25.77
Granted, weighted average grant date fair value	$ 30.60
Vested, weighted average grant date fair value	$ 24.80
Forfeited, weighted average grant date fair value	$ 28.05
Unvested at end of period, weighted average grant date fair value	$ 28.93	$ 25.77
Performance units
Summary of awards:
Granted (in Shares)	572	656	1,527
Potential maximum share payout	1,144	1,312	3,054
Aggregate grant date fair value	16,345,000	17,753,000	29,537,000
Compensation expense	24,089,000	1,041,000	9,919,000
Estimated tax benefit (charge)	4,250,000	(176,000)	1,943,000
Total unrecognized stock based compensation expense	$ 27,800,000
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	1,687
Granted, shares	572	656	1,527
Vested, shares	(340)
Forfeited, shares	(100)
Performance driven reduction, shares	721
Unvested at end of period, shares	2,540	1,687
Unvested at beginning of period, weighted average grant date fair value	$ 22.99
Granted, weighted average grant date fair value	$ 28.57
Vested, weighted average grant date fair value	$ 22.06
Forfeited, weighted average grant date fair value	$ 25.98
Performance driven reduction, weighted average grant date fair value	$ 26.46
Unvested at end of period, weighted average grant date fair value	$ 25.24	$ 22.99
Performance units | Minimum
Summary of awards:
Final payout percentage	0.00%
Performance units | Maximum
Summary of awards:
Final payout percentage	200.00%