Consolidated Statement of Changes in Common Shareholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	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)
Balance (in shares) at Dec. 31, 2014	(629,819)		(629,819)
Balance (in shares) at Dec. 31, 2014	61,051,286	61,051,286
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Common stock issued for compensatory plans	742	8	2,069	(1,335)
Common stock issued for compensatory plans (in shares)		7,632	51,487
Treasury Stock Acquired [Abstract]
Dividends on common stock	(24,343)				(24,343)
Net income	26,922				26,922
Other comprehensive income (loss), net of tax	662					662
Balances at Mar. 31, 2015	$ 1,631,253	$ 61,059	$ (23,241)	$ 414,147	$ 1,211,291	$ (32,003)
Balance (in shares) at Mar. 31, 2015	61,058,918	61,058,918
Balance (in shares) at Mar. 31, 2015	(578,332)		(578,332)