Consolidated Statement of Changes in Common Shareholders' Equity - 6 months ended Jun. 30, 2015 - 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, 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	2,228	8	2,085	135
Dividends on common stock	(48,685)				(48,685)
Net income	57,156				57,156
Other comprehensive income, net of tax	1,371					1,371
Balances at Jun. 30, 2015	$ 1,639,340	$ 61,059	$ (23,225)	$ 415,617	$ 1,217,183	$ (31,294)
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	51,879
Common stock, balance (in shares) at Jun. 30, 2015	61,058,918	61,058,918
Treasury stock, balance (in shares) at Jun. 30, 2015	(577,940)		(577,940)