Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating revenues	$ 280,773	$ 235,866	$ 244,603	$ 309,100	$ 283,209	$ 244,041	$ 251,806	$ 338,260	$ 1,070,342	$ 1,117,316	$ 1,110,720
Operating income	89,024	4,409	28,720	55,033	49,332	31,878	26,244	58,095	177,185	165,549	162,617
Net Income	$ 58,697	$ (3,772)	$ 11,438	$ 32,043	$ 34,116	$ 14,895	$ 10,970	$ 32,575	$ 98,406	$ 92,556	$ 77,376
Average Common Shares Outstanding	37,218,000	37,201,000	36,635,000	36,328,000	36,271,000	36,262,000	36,258,000	36,242,000	36,847,427	36,258,463	36,190,373
Income per average common share, (basic)
Net income (loss)	$ 1.58	$ (0.10)	$ 0.31	$ 0.88	$ 0.94	$ 0.41	$ 0.30	$ 0.90	$ 2.67	$ 2.55	$ 2.14
Income per average common share (diluted)
Net income (loss)	$ 1.57	$ (0.10)	$ 0.31	$ 0.88	$ 0.93	$ 0.41	$ 0.30	$ 0.89	$ 2.66	$ 2.53	$ 2.14
Dividends per share	$ 0.37	$ 0.37	$ 0.37	$ 0.37	$ 0.36	$ 0.36	$ 0.36	$ 0.36	$ 1.48	$ 1.44	$ 1.36
Quarter-end close	$ 34.73	$ 36.23	$ 36.70	$ 35.46	$ 35.79	$ 31.94	$ 33.11	$ 30.30	$ 34.73	$ 35.79
Maximum
Income per average common share (diluted)
Stock price	$ 36.70	$ 37.96	$ 37.05	$ 36.39	$ 36.61	$ 34.17	$ 33.24	$ 30.57
Minimum
Income per average common share (diluted)
Stock price	$ 32.98	$ 35.44	$ 33.72	$ 34.22	$ 30.44	$ 28.68	$ 29.37	$ 27.38