Summarized Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 256,160	$ 231,046	$ 228,114	$ 285,142	$ 240,388	$ 231,060	$ 210,175	$ 241,415	$ 1,000,462	$ 923,038	$ 861,842
Operating income	27,787	37,540	23,730	42,571	16,545	40,769	22,466	37,401	131,628	117,181	99,866
Net income	$ 14,355	$ 20,030	$ 12,022	$ 24,235	$ 7,225	$ 22,013	$ 10,544	$ 19,685	$ 70,642	$ 59,468	$ 49,951