Consolidated Statements of Equity and Redeemable Non-controlling Interest - USD ($) $ in Thousands	Total	Common shares	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning balance, value at Dec. 31, 2017	$ 1,424,044	$ 1,924	$ 1,421,368	$ 355,982	$ (355,230)
Beginning balance, value (in shares) at Dec. 31, 2017		192,825,207
Redeemable non-controlling interest, Beginning balance at Dec. 31, 2017	4,750
Adoption of ASU (ASU 2014-09) at Dec. 31, 2017	17,924			17,924
Adjusted balance, value at Dec. 31, 2017	1,441,968	$ 1,924	1,421,368	373,906	(355,230)
Redeemable non-controlling interest, Adjusted balance at Dec. 31, 2017	4,750
Adoption of ASU | ASU 2018-02				(2,265)	2,265
Issuance of common shares on exercise of options	6,211	$ 4	6,207
Issuance of common shares on exercise of options (in shares)		366,382
Issuance of common shares under the employee stock purchase plan	3,177	$ 1	3,176
Issuance of common shares under the employee stock purchase plan (in shares)		114,951
Net settlement on vesting of restricted share units	(945)	$ 2	(947)
Net settlement on vesting of restricted share units (in shares)		156,420
Net settlement on vesting of performance units	(13,284)	$ 7	(13,291)
Net settlement on vesting of performance units (in shares)		691,958
Stock repurchased and retired	$ (130,103)	$ (43)	4,000	(134,060)
Stock repurchased and retired (in shares)	(4,114,882)	(4,278,857)
Expenses related to stock repurchase	$ (82)			(82)
Stock-based compensation expense	18,724		18,724
Payment for acquisition of redeemable non-controlling interest	(1,165)		(1,165)
Redeemable non-controlling interest, Acquisition of redeemable non controlling interest	(3,565)
Comprehensive income (loss):
Net income (loss)	129,269			129,269
Redeemable non-controlling interest, Net income (loss)	(761)
Other comprehensive income (loss)	(128,697)				(128,697)
Redeemable non-controlling interest, Other comprehensive income (loss)	(424)
Dividend	(28,648)			(28,648)
End balance, value at Jun. 30, 2018	1,296,425	$ 1,895	1,438,072	338,120	(481,662)
End balance, value (in shares) at Jun. 30, 2018		189,876,061
Beginning balance, value at Mar. 31, 2018	1,365,997	$ 1,903	1,422,897	321,916	(380,719)
Beginning balance, value (in shares) at Mar. 31, 2018		190,613,135
Issuance of common shares on exercise of options	3,660	$ 2	3,658
Issuance of common shares on exercise of options (in shares)		204,545
Issuance of common shares under the employee stock purchase plan	1,526		1,526
Issuance of common shares under the employee stock purchase plan (in shares)		56,475
Net settlement on vesting of restricted share units	(945)	$ 1	(946)
Net settlement on vesting of restricted share units (in shares)		100,789
Stock repurchased and retired	(34,119)	$ (11)		(34,108)
Stock repurchased and retired (in shares)		(1,098,883)
Expenses related to stock repurchase	(22)			(22)
Stock-based compensation expense	10,937		10,937
Comprehensive income (loss):
Net income (loss)	64,574			64,574
Other comprehensive income (loss)	(100,943)
Dividend	(14,240)			(14,240)
Other comprehensive income (loss)	(100,943)				(100,943)
End balance, value at Jun. 30, 2018	1,296,425	$ 1,895	1,438,072	338,120	(481,662)
End balance, value (in shares) at Jun. 30, 2018		189,876,061
Beginning balance, value at Dec. 31, 2018	$ 1,404,182	$ 1,888	1,471,301	438,453	(507,460)
Beginning balance, value (in shares) at Dec. 31, 2018	189,346,101	189,346,101
Issuance of common shares on exercise of options	$ 7,277	$ 5	7,272
Issuance of common shares on exercise of options (in shares)	506,497	506,497
Issuance of common shares under the employee stock purchase plan	$ 4,200	$ 1	4,199
Issuance of common shares under the employee stock purchase plan (in shares)		134,346
Net settlement on vesting of restricted share units	(2,729)	$ 5	(2,734)
Net settlement on vesting of restricted share units (in shares)		499,097
Stock-based compensation expense	39,987		39,987
Comprehensive income (loss):
Net income (loss)	134,563			134,563
Other comprehensive income (loss)	28,197
Dividend	(32,307)			(32,307)
Other comprehensive income (loss)	28,197				28,197
End balance, value at Jun. 30, 2019	$ 1,583,370	$ 1,899	1,520,025	540,709	(479,263)
End balance, value (in shares) at Jun. 30, 2019	190,486,041	190,486,041
Beginning balance, value at Mar. 31, 2019	$ 1,495,169	$ 1,891	1,493,706	483,175	(483,603)
Beginning balance, value (in shares) at Mar. 31, 2019		189,659,709
Issuance of common shares on exercise of options	4,618	$ 3	4,615
Issuance of common shares on exercise of options (in shares)		370,962
Issuance of common shares under the employee stock purchase plan	2,261	$ 1	2,260
Issuance of common shares under the employee stock purchase plan (in shares)		69,477
Net settlement on vesting of restricted share units	(2,077)	$ 4	(2,081)
Net settlement on vesting of restricted share units (in shares)		385,893
Stock-based compensation expense	21,525		21,525
Comprehensive income (loss):
Net income (loss)	73,722			73,722
Other comprehensive income (loss)	4,340
Dividend	(16,188)			(16,188)
Other comprehensive income (loss)	4,340				4,340
End balance, value at Jun. 30, 2019	$ 1,583,370	$ 1,899	$ 1,520,025	$ 540,709	$ (479,263)
End balance, value (in shares) at Jun. 30, 2019	190,486,041	190,486,041