Consolidated Statement of Changes in Common Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Premium on Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balances at Dec. 31, 2013						$ (25,876)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 133,392
Other comprehensive income, net of tax	1,759					1,759
Balances at Sep. 30, 2014						(24,117)
Balances at Jun. 30, 2014						(24,488)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	70,835
Other comprehensive income, net of tax	371					371
Balances at Sep. 30, 2014						(24,117)
Balances at Dec. 31, 2014	1,627,270	$ 61,051	$ (25,310)	$ 415,482	$ 1,208,712	(32,665)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued for compensatory plans	3,676	8	2,128	1,540
Dividends on common stock	(73,025)				(73,025)
Net income	111,819				111,819
Other comprehensive income, net of tax	2,010					2,010
Balances at Sep. 30, 2015	$ 1,671,750	$ 61,059	$ (23,182)	417,022	1,247,506	(30,655)
Common stock, balance (in shares) at Dec. 31, 2014	61,051,286	61,051,286
Treasury stock, balance (in shares) at Dec. 31, 2014	(629,819)		(629,819)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued for compensatory plans (in shares)		7,632	52,952
Common stock, balance (in shares) at Sep. 30, 2015	61,058,918	61,058,918
Treasury stock, balance (in shares) at Sep. 30, 2015	(576,867)		(576,867)
Balances at Jun. 30, 2015						(31,294)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 54,663
Other comprehensive income, net of tax	639					639
Balances at Sep. 30, 2015	$ 1,671,750	$ 61,059	$ (23,182)	$ 417,022	$ 1,247,506	$ (30,655)
Common stock, balance (in shares) at Sep. 30, 2015	61,058,918	61,058,918
Treasury stock, balance (in shares) at Sep. 30, 2015	(576,867)		(576,867)