Statements of Equity (USD $) In Thousands, unless otherwise specified	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (loss) [Member]	Total Equity
Balance at Dec. 31, 2013		$ 2,500	$ 901,422	$ 764,846	$ 70,867	$ 1,739,635
Comprehensive income:
Net income	77,498	0	0	77,498	0	77,498
Other comprehensive income (loss), net of tax	12,599	0	0	0	12,599	12,599
Total comprehensive income (loss)						90,097
Balance at Mar. 31, 2014		2,500	901,422	842,344	83,466	1,829,732
Balance at Dec. 31, 2014		2,500	901,422	673,613	84,622	1,662,157
Comprehensive income:
Net income	(21,302)	0	0	(21,302)	0	(21,302)
Other comprehensive income (loss), net of tax	8,367	0	0	0	8,367	8,367
Total comprehensive income (loss)						(12,935)
Balance at Mar. 31, 2015		$ 2,500	$ 901,422	$ 652,311	$ 92,989	$ 1,649,222