Capital Stock - Narrative (Details) (USD $)	6 Months Ended								1 Months Ended	6 Months Ended	0 Months Ended	6 Months Ended
	Jun. 30, 2014 common_share director	Jun. 30, 2013	Jun. 30, 2014 Minimum	Jun. 30, 2013 Minimum	Jun. 30, 2014 Maximum	Jun. 30, 2013 Maximum	Jun. 30, 2014 Common stock, Class A	Jun. 30, 2013 Common stock, Class A	Nov. 30, 2012 First Repurchase Plan Common stock, Class A	Jun. 30, 2014 First Repurchase Plan Common stock, Class A	May 06, 2014 Second Repurchase Plan Common stock, Class A	Jun. 30, 2014 Second 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		$ 100,000,000
Stock repurchased during period, value	21,237,000	34,910,000					21,200,000	34,900,000				0
Range of price of shares repurchased (USD per share)			$ 16.35	$ 10.83	$ 19.99	$ 15.49
Stock Repurchase Program, Remaining Authorized Repurchase Amount										$ 5,000,000