Capital Stock (Details)		6 Months Ended
	Jan. 05, 2017 USD ($)	Jun. 30, 2017 USD ($) director common_share $ / shares	Jun. 30, 2016 USD ($) $ / shares	Nov. 30, 2016 USD ($)	May 31, 2014 USD ($)
Class of Stock [Line Items]
Classes of common shares | common_share		2
Stock repurchased during period, value		$ 6,257,000	$ 18,686,000
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
First Repurchase Plan [Member] | Common stock, Class A
Class of Stock [Line Items]
Stock repurchase program, authorized amount					$ 100,000,000
Stock repurchased during period, value	$ 500,000		$ 18,700,000
Stock repurchase program, remaining authorized amount		$ 0
First Repurchase Plan [Member] | Common stock, Class A | Minimum
Class of Stock [Line Items]
Stock repurchased during period (in dollars per share) | $ / shares			$ 14.71
First Repurchase Plan [Member] | Common stock, Class A | Maximum
Class of Stock [Line Items]
Stock repurchased during period (in dollars per share) | $ / shares			$ 19.51
Second Repurchase Plan [Member] | Common stock, Class A
Class of Stock [Line Items]
Stock repurchase program, authorized amount				$ 100,000,000
Stock repurchased during period, value		6,300,000
Stock repurchase program, remaining authorized amount		$ 94,200,000
Second Repurchase Plan [Member] | Common stock, Class A | Minimum
Class of Stock [Line Items]
Stock repurchased during period (in dollars per share) | $ / shares		$ 16.82
Second Repurchase Plan [Member] | Common stock, Class A | Maximum
Class of Stock [Line Items]
Stock repurchased during period (in dollars per share) | $ / shares		$ 23.01