CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common shares par value [Member]	Additional paid-in capital [Member]	Retained earnings [Member]	Accumulated other comprehensive loss [Member]
Balance, beginning of year at Dec. 31, 2017	$ 1,015,013	$ 47,953	$ 0	$ 1,004,657	$ (37,597)
Balance (in shares) at Dec. 31, 2017	47,953
Net (loss) income	$ 92,461			92,461
Cash dividends (quarters ended September 30, 2019 and 2018 - $0.30 per share; nine months ended September 30, 2019 and 2018 - $0.90 per share)	(43,012)			(43,012)
Common shares issued	$ 18,699	518	18,181
Common shares issued (in shares)	518
Common shares repurchased	$ (120,000)	(1,919)	(10,121)	(107,960)
Common shares repurchased (in shares)	(1,919)
Other common shares retired	$ (17,848)	(247)	(17,601)
Other common shares retired (in shares)	(247)
Employee share-based compensation	$ 9,541		9,541
Other comprehensive (loss) income	(4,525)				(4,525)
Balance, end of period at Sep. 30, 2018	$ 955,295	46,305	0	957,979	(48,989)
Balance (in shares) at Sep. 30, 2018	46,305
Balance, beginning of year at Jun. 30, 2018	$ 1,075,136	47,623	0	1,076,683	(49,170)
Balance (in shares) at Jun. 30, 2018	47,623
Net (loss) income	$ (31,083)			(31,083)
Cash dividends (quarters ended September 30, 2019 and 2018 - $0.30 per share; nine months ended September 30, 2019 and 2018 - $0.90 per share)	(14,209)			(14,209)
Common shares issued	$ 1,532	28	1,504
Common shares issued (in shares)	28
Common shares repurchased	$ (80,004)	(1,346)	(5,246)	(73,412)
Common shares repurchased (in shares)	(1,346)
Other common shares retired	$ (22)	0	(22)
Other common shares retired (in shares)	0
Employee share-based compensation	$ 3,764		3,764
Other comprehensive (loss) income	181				181
Balance, end of period at Sep. 30, 2018	$ 955,295	46,305	0	957,979	(48,989)
Balance (in shares) at Sep. 30, 2018	46,305
Adoption of new accounting pronouncement | Accounting Standards Update No. 2014-09 [Member]	$ 4,966			4,966
Adoption of new accounting pronouncement | Adoption of Accounting Standards Update No. 2018-02 [Member]	0			6,867	(6,867)
Balance, beginning of year at Dec. 31, 2018	$ 915,413	44,647	0	927,345	(56,579)
Balance (in shares) at Dec. 31, 2018	44,647
Net (loss) income	$ (244,721)			(244,721)
Cash dividends (quarters ended September 30, 2019 and 2018 - $0.30 per share; nine months ended September 30, 2019 and 2018 - $0.90 per share)	(39,445)			(39,445)
Common shares issued	$ 3,561	150	3,411
Common shares issued (in shares)	150
Common shares repurchased	$ (118,547)	(2,632)	(13,615)	(102,300)
Common shares repurchased (in shares)	(2,632)
Other common shares retired	$ (3,076)	(66)	(3,010)
Other common shares retired (in shares)	(66)
Employee share-based compensation	$ 13,214		13,214
Other comprehensive (loss) income	(605)				(605)
Balance, end of period at Sep. 30, 2019	$ 525,527	42,099	0	540,612	(57,184)
Balance (in shares) at Sep. 30, 2019	42,099
Balance, beginning of year at Jun. 30, 2019	$ 894,149	42,928	0	904,748	(53,527)
Balance (in shares) at Jun. 30, 2019	42,928
Net (loss) income	$ (318,493)			(318,493)
Cash dividends (quarters ended September 30, 2019 and 2018 - $0.30 per share; nine months ended September 30, 2019 and 2018 - $0.90 per share)	(12,977)			(12,977)
Common shares issued	$ 1,523	51	1,472
Common shares issued (in shares)	51
Common shares repurchased	$ (39,651)	(876)	(6,109)	(32,666)
Common shares repurchased (in shares)	(876)
Other common shares retired	$ (204)	(4)	(200)
Other common shares retired (in shares)	(4)
Employee share-based compensation	$ 4,837		4,837
Other comprehensive (loss) income	(3,657)				(3,657)
Balance, end of period at Sep. 30, 2019	$ 525,527	$ 42,099	$ 0	540,612	$ (57,184)
Balance (in shares) at Sep. 30, 2019	42,099
Adoption of new accounting pronouncement | Adoption of Accounting Standards Update No. 2016-02 (Note 2) [Member]	$ (267)			$ (267)