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	159,661
Treasury shares acquired, shares	(434,878)
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		5,170
Net increase (decrease) during the period			4,652			$ 4,652
Net income (loss)				322,978		322,978
Dividends declared ($1.15 per share in 2016 and $0.95 per share in 2015)				(42,252)		(42,252)
Purchase of treasury shares					(75,040)
Balance, end of period, shares at Mar. 31, 2015	44,410,420
Balance, end of period at Mar. 31, 2015	$ 685	2,073,977	52,969	8,099,936	(2,560,937)	$ 7,666,630
Balance, beginning of period, shares at Dec. 31, 2015	42,694,252					68,606,000
Issued during the period, shares	169,774
Treasury shares acquired, shares	(464,360)
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		8,190
Net increase (decrease) during the period			186,456			186,456
Net income (loss)				171,686		171,686
Dividends declared ($1.15 per share in 2016 and $0.95 per share in 2015)				(48,706)		$ (48,706)
Purchase of treasury shares					(85,914)
Balance, end of period, shares at Mar. 31, 2016	42,399,666					68,776,000
Balance, end of period at Mar. 31, 2016	$ 688	$ 2,111,828	$ (45,299)	$ 8,744,952	$ (2,971,870)	$ 7,840,299