Earnings per common share (Net Income, Weighted Average Common Shares Used In Calculating Basic Earnings Per Common Share And Diluted EPS) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator
Net income	$ 65,744		$ (7,022)		$ 32,789		$ 50,044		$ 49,755	$ 140,297	$ 143,610	$ 137,245	$ 141,555	$ 470,907	$ 488,907
Dividends													(94,168)	(77,495)	(74,128)
Undistributed earnings													$ 47,387	$ 393,412	$ 414,779
Denominator
Weighted average shares outstanding used in basic earnings per share calculations													68,163,825	72,181,447	76,648,688
Weighted average shares used in diluted earnings per share calculations													69,017,209	73,152,010	77,654,764
Earnings per common share – Basic
Distributed earnings - Basic (in dollars per share)													$ 1.38	$ 1.06	$ 0.96
Undistributed earnings - Basic (in dollars per share)													0.70	5.46	5.42
Net income, basic (in dollars per share)	$ 0.99		$ (0.10)		$ 0.48		$ 0.72		$ 0.70	$ 1.94	$ 1.98	$ 1.88	2.08	6.52	6.38
Earnings per common share – Diluted
Distributed earnings - Diluted (in dollars per share)													1.36	1.06	0.95
Undistributed earnings - Diluted (in dollars per share)													0.69	5.38	5.35
Net income - Diluted (in dollars per share)	$ 0.97	[1]	$ (0.10)	[1]	$ 0.47	[1]	$ 0.71	[1]	$ 0.69	$ 1.92	$ 1.95	$ 1.86	$ 2.05	$ 6.44	$ 6.30
Retained Earnings
Numerator
Net income													$ 141,555	$ 470,907	$ 488,907
Dividends													$ (94,168)	$ (77,495)	$ (74,128)
PSUs
Denominator
Incremental common shares													789,547	905,648	864,572
SARs
Denominator
Incremental common shares													0	0	65,712
ESPP
Denominator
Incremental common shares													63,837	64,915	75,792

[1] In accordance with earnings per share guidance, diluted per share amounts are computed in the same manner as basic per share amounts when a loss from operations exists.