Capital Stock - Narrative (Details)	9 Months Ended
	Sep. 30, 2015 USD ($) director common_share $ / shares	Sep. 30, 2014 USD ($)	May. 31, 2014 USD ($)
Class of Stock [Line Items]
Classes of common shares | common_share	2
Minimum number of directors up for election to entitle shareholders to vote | director	3
Stock repurchased during period, value	$ 10,901,000	$ 21,237,000
Common stock, Class A
Class of Stock [Line Items]
Stock repurchase program, authorized amount			$ 100,000,000
Stock repurchased during period, value	10,900,000
Stock repurchase program, remaining authorized amount	$ 89,100,000
Common stock, Class A | Minimum
Class of Stock [Line Items]
Stock repurchased during period (in dollars per share) | $ / shares	$ 15.90
Common stock, Class A | Maximum
Class of Stock [Line Items]
Stock repurchased during period (in dollars per share) | $ / shares	$ 24.08