Consolidated Statements Of Changes In Shareholders' Equity - USD ($) $ in Thousands	Common Shares [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income (Loss), Net Of Deferred Income Taxes [Member]	Retained Earnings [Member]	Treasury Shares At Cost [Member]	Total
Balance, beginning of period, shares at Dec. 31, 2014	44,685,637
Issued during the period, shares	219,267
Treasury shares acquired, shares	(712,378)
Balance, beginning of period at Dec. 31, 2014	$ 683	$ 2,068,807	$ 48,317	$ 7,819,210	$ (2,485,897)
Issued during the period, net	$ 2
Share-based compensation plans		15,829
Net increase (decrease) during the period			(63,136)			$ (63,136)
Net income (loss)				532,035		532,035
Dividends declared ($1.15 per share in second quarter 2016 and $2.30 year-to-date per share in 2016 and $0.95 per share in second quarter 2015 and $1.90 year-to-date per share in 2015)				(84,207)		(84,207)
Purchase of treasury shares					(124,981)
Balance, end of period, shares at Jun. 30, 2015	44,192,526
Balance, end of period at Jun. 30, 2015	$ 685	2,084,636	(14,819)	8,267,038	(2,610,878)	7,726,662
Balance, beginning of period, shares at Mar. 31, 2015	44,410,420
Issued during the period, shares	59,606
Treasury shares acquired, shares	(277,500)
Balance, beginning of period at Mar. 31, 2015	$ 685	2,073,977	52,969	8,099,936	(2,560,937)
Share-based compensation plans		10,659
Net increase (decrease) during the period			(67,788)			(67,788)
Net income (loss)				209,057		209,057
Dividends declared ($1.15 per share in second quarter 2016 and $2.30 year-to-date per share in 2016 and $0.95 per share in second quarter 2015 and $1.90 year-to-date per share in 2015)				(41,955)		(41,955)
Purchase of treasury shares					(49,941)
Balance, end of period, shares at Jun. 30, 2015	44,192,526
Balance, end of period at Jun. 30, 2015	$ 685	2,084,636	(14,819)	8,267,038	(2,610,878)	$ 7,726,662
Balance, beginning of period, shares at Dec. 31, 2015	42,694,252					68,606,000
Issued during the period, shares	198,266
Treasury shares acquired, shares	(1,009,088)
Balance, beginning of period at Dec. 31, 2015	$ 686	2,103,638	(231,755)	8,621,972	(2,885,956)	$ 7,608,585
Issued during the period, net	$ 2
Share-based compensation plans		16,943
Net increase (decrease) during the period			315,755			315,755
Net income (loss)				327,378		327,378
Dividends declared ($1.15 per share in second quarter 2016 and $2.30 year-to-date per share in 2016 and $0.95 per share in second quarter 2015 and $1.90 year-to-date per share in 2015)				(96,838)		$ (96,838)
Purchase of treasury shares					(186,357)
Balance, end of period, shares at Jun. 30, 2016	41,883,430					68,805,000
Balance, end of period at Jun. 30, 2016	$ 688	2,120,581	84,000	8,852,512	(3,072,313)	$ 7,985,468
Balance, beginning of period, shares at Mar. 31, 2016	42,399,666
Issued during the period, shares	28,492
Treasury shares acquired, shares	(544,728)
Balance, beginning of period at Mar. 31, 2016	$ 688	2,111,828	(45,299)	8,744,952	(2,971,870)
Share-based compensation plans		8,753
Net increase (decrease) during the period			129,299			129,299
Net income (loss)				155,692		155,692
Dividends declared ($1.15 per share in second quarter 2016 and $2.30 year-to-date per share in 2016 and $0.95 per share in second quarter 2015 and $1.90 year-to-date per share in 2015)				(48,132)		$ (48,132)
Purchase of treasury shares					(100,443)
Balance, end of period, shares at Jun. 30, 2016	41,883,430					68,805,000
Balance, end of period at Jun. 30, 2016	$ 688	$ 2,120,581	$ 84,000	$ 8,852,512	$ (3,072,313)	$ 7,985,468