Share Capital (Details) - Restricted stock, restricted stock units and performance units (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Minimum
Award activity:
Vesting period required	3 years
Maximum
Award activity:
Vesting period required	4 years
Restricted stock awards
Summary of awards:
Granted (in Shares)	325	49	63
Weighted average grant date fair value (per share)	$ 20.92	$ 23.33	$ 17.26
Aggregate grant date fair value	$ 6,811,000	$ 1,140,000	$ 1,080,000
Compensation expense	5,399,000	7,101,000	18,420,000
Estimated tax benefit	1,113,000	1,546,000	4,645,000
Award activity:
Unvested at beginning of period, shares	390
Granted, shares	325	49	63
Vested, shares	(161)
Forfeited, shares	(4)
Unvested at end of period, shares	550	390
Unvested at beginning of period, weighted average grant date fair value	$ 42.86
Granted, weighted average grant date fair value	$ 20.92
Vested, weighted average grant date fair value	$ 31.08
Forfeited, weighted average grant date fair value	$ 43.06
Unvested at end of period, weighted average grant date fair value	$ 33.34	$ 42.86
Total unrecognized stock based compensation expense	5,800,000
Period for recognition of unrecognized expense	1 year 4 months 24 days
Restricted stock units
Summary of awards:
Granted (in Shares)	1,312	1,318	1,392
Aggregate grant date fair value	27,194,000	30,027,000	25,887,000
Compensation expense	24,451,000	15,033,000	5,862,000
Estimated tax benefit	6,501,000	3,998,000	1,557,000
Award activity:
Unvested at beginning of period, shares	2,063
Granted, shares	1,312	1,318	1,392
Vested, shares	(836)
Forfeited, shares	(93)
Unvested at end of period, shares	2,446	2,063
Unvested at beginning of period, weighted average grant date fair value	$ 21.13
Granted, weighted average grant date fair value	$ 20.72
Vested, weighted average grant date fair value	$ 20.73
Forfeited, weighted average grant date fair value	$ 21.42
Unvested at end of period, weighted average grant date fair value	$ 21.04	$ 21.13
Total unrecognized stock based compensation expense	32,500,000
Period for recognition of unrecognized expense	1 year 3 months 18 days
Vesting period required	3 years
Performance units
Summary of awards:
Granted (in Shares)	1,527	1,289	1,579
Potential maximum share payout, shares	3,054	2,578	3,158
Aggregate grant date fair value	29,537,000	28,544,000	27,160,000
Compensation expense	9,919,000	7,242,000	7,009,000
Estimated tax benefit	1,943,000	1,419,000	1,470,000
Award activity:
Unvested at beginning of period, shares	1,735
Granted, shares	1,527	1,289	1,579
Vested, shares	(7)
Forfeited, shares	(98)
Performance driven reduction, shares	(601)
Unvested at end of period, shares	2,556	1,735
Unvested at beginning of period, weighted average grant date fair value	$ 19.47
Granted, weighted average grant date fair value	$ 19.34
Vested, weighted average grant date fair value	$ 17.20
Forfeited, weighted average grant date fair value	$ 20.34
Performance driven reduction, weighted average grant date fair value	$ 18.24
Unvested at end of period, weighted average grant date fair value	$ 20.07	$ 19.47
Total unrecognized stock based compensation expense	$ 27,200,000
Period for recognition of unrecognized expense	1 year 10 months 24 days
Vesting period required	3 years
Performance units | Minimum
Award activity:
Final payout percentage	0.00%
Performance units | Maximum
Award activity:
Final payout percentage	200.00%