Capital Stock - Narrative (Details) (USD $)	6 Months Ended					1 Months Ended
	Jun. 30, 2013 common_share director	Jun. 30, 2012	Jun. 30, 2013 Minimum	Jun. 30, 2013 Maximum	Jun. 30, 2013 Common stock, Class A	Nov. 30, 2012 First Repurchase Plan Common stock, Class A
Class of Stock [Line Items]
Classes of common shares	2
Minimum number of directors up for election to entitle shareholders to vote	3
Minumum percent of directors up for election to entitle shareholders to vote	33.33%
Stock repurchase program, authorized amount						$ 100,000,000
Stock repurchased during period, value	$ 34,910,000	$ 16,407,000			$ 34,900,000
Range of price of shares repurchased (USD per share)			$ 10.83	$ 15.49