CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 18,941	$ 42,860	$ 70,930	$ 149,933
Other comprehensive income, net of tax:
Net actuarial pension loss amortization, net of income tax of $196, $265, $589 and $796	310	414	929	1,241
Prior service (credit) amortization, net of income tax of $151, $152, $453 and $455	(238)	(237)	(714)	(711)
Unrealized gains (losses) on available-for-sale securities, net of income tax of $0, $0, $0 and $0	0	0	0	(21)
Other comprehensive income, net of tax	72	177	215	509
Comprehensive Income	$ 19,013	$ 43,037	$ 71,145	$ 150,442