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	7,258	$ 4	7,254
Issuance of common shares on exercise of options (in shares)		441,076
Issuance of common shares under the employee stock purchase plan	5,017	$ 2	5,015
Issuance of common shares under the employee stock purchase plan (in shares)		181,643
Net settlement on vesting of restricted share units	(1,663)	$ 2	(1,665)
Net settlement on vesting of restricted share units (in shares)		192,222
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	32,158		32,158
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)	202,872			202,872
Redeemable non-controlling interest, Net income (loss)	(761)
Other comprehensive income (loss)	(215,057)				(215,057)
Redeemable non-controlling interest, Other comprehensive income (loss)	(424)
Dividend	(42,901)			(42,901)
End balance, value at Sep. 30, 2018	1,285,018	$ 1,896	1,453,674	397,470	(568,022)
End balance, value (in shares) at Sep. 30, 2018		190,053,249
Beginning balance, value at Jun. 30, 2018	1,296,425	$ 1,895	1,438,072	338,120	(481,662)
Beginning balance, value (in shares) at Jun. 30, 2018		189,876,061
Issuance of common shares on exercise of options	1,047		1,047
Issuance of common shares on exercise of options (in shares)		74,694
Issuance of common shares under the employee stock purchase plan	1,840	$ 1	1,839
Issuance of common shares under the employee stock purchase plan (in shares)		66,692
Net settlement on vesting of restricted share units	(718)		(718)
Net settlement on vesting of restricted share units (in shares)		35,802
Stock-based compensation expense	13,434		13,434
Comprehensive income (loss):
Net income (loss)	73,603			73,603
Other comprehensive income (loss)	(86,360)
Other comprehensive income (loss)	(86,360)				(86,360)
Dividend	(14,253)			(14,253)
End balance, value at Sep. 30, 2018	1,285,018	$ 1,896	1,453,674	397,470	(568,022)
End balance, value (in shares) at Sep. 30, 2018		190,053,249
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	$ 9,505	$ 6	9,499
Issuance of common shares on exercise of options (in shares)	622,494	622,494
Issuance of common shares under the employee stock purchase plan	$ 6,444	$ 2	6,442
Issuance of common shares under the employee stock purchase plan (in shares)		195,221
Net settlement on vesting of restricted share units	(3,788)	$ 6	(3,794)
Net settlement on vesting of restricted share units (in shares)		557,917
Stock repurchased and retired	$ (23,901)	$ (6)		(23,895)
Stock repurchased and retired (in shares)	(608,285)	(608,285)
Expenses related to stock repurchase	$ (12)			(12)
Stock-based compensation expense	61,307		61,307
Comprehensive income (loss):
Net income (loss)	222,683			222,683
Other comprehensive income (loss)	(22,221)
Other comprehensive income (loss)	(22,221)				(22,221)
Dividend	(48,515)			(48,515)
End balance, value at Sep. 30, 2019	$ 1,605,684	$ 1,896	1,544,755	588,714	(529,681)
End balance, value (in shares) at Sep. 30, 2019	190,113,448	190,113,448
Beginning balance, value at Jun. 30, 2019	$ 1,583,370	$ 1,899	1,520,025	540,709	(479,263)
Beginning balance, value (in shares) at Jun. 30, 2019		190,486,041
Issuance of common shares on exercise of options	2,228	$ 1	2,227
Issuance of common shares on exercise of options (in shares)		115,997
Issuance of common shares under the employee stock purchase plan	2,244	$ 1	2,243
Issuance of common shares under the employee stock purchase plan (in shares)		60,875
Net settlement on vesting of restricted share units	(1,059)	$ 1	(1,060)
Net settlement on vesting of restricted share units (in shares)		58,820
Stock repurchased and retired	(23,901)	$ (6)		(23,895)
Stock repurchased and retired (in shares)		(608,285)
Expenses related to stock repurchase	(12)			(12)
Stock-based compensation expense	21,320		21,320
Comprehensive income (loss):
Net income (loss)	88,120			88,120
Other comprehensive income (loss)	(50,418)
Other comprehensive income (loss)	(50,418)				(50,418)
Dividend	(16,208)			(16,208)
End balance, value at Sep. 30, 2019	$ 1,605,684	$ 1,896	$ 1,544,755	$ 588,714	$ (529,681)
End balance, value (in shares) at Sep. 30, 2019	190,113,448	190,113,448