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, 2012		$ 2,500	$ 893,336	$ 200,754	$ 147,287	$ 1,243,877
Affiliated asset transfers		0	8,086	0	0	8,086
Comprehensive income:
Net income	147,388	0	0	147,388	0	147,388
Other comprehensive income (loss), net of tax	(9,117)	0	0	0	(9,117)	(9,117)
Total comprehensive income (loss)						138,271
Balance at Mar. 31, 2013		2,500	901,422	348,142	138,170	1,390,234
Balance at Dec. 31, 2012		2,500	893,336	200,754	147,287	1,243,877
Comprehensive income:
Net income
Other comprehensive income (loss), net of tax
Total comprehensive income (loss)
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