Capital Stock - Narrative (Details)	6 Months Ended
	Jun. 30, 2016 USD ($) director common_share $ / shares	Jun. 30, 2015 USD ($) $ / shares	May 31, 2014 USD ($)
Class of Stock [Line Items]
Classes of common shares | common_share	2
Stock repurchased during period, value	$ 18,686,000	$ 2,683,000
Minimum
Class of Stock [Line Items]
Minimum number of directors up for election to entitle shareholders to vote | director	0.3333
Maximum
Class of Stock [Line Items]
Minimum number of directors up for election to entitle shareholders to vote | director	3
Common stock, Class A
Class of Stock [Line Items]
Stock repurchase program, authorized amount			$ 100,000,000
Stock repurchased during period, value	$ 18,700,000	$ 2,700,000
Stock repurchase program, remaining authorized amount	$ 65,100,000
Common stock, Class A | Minimum
Class of Stock [Line Items]
Stock repurchased during period (in dollars per share) | $ / shares	$ 14.71	$ 21.60
Common stock, Class A | Maximum
Class of Stock [Line Items]
Stock repurchased during period (in dollars per share) | $ / shares	$ 19.51	$ 24.94