Statement of Equity (USD $) In Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balance at Dec. 31, 2009	$ 3,467,147	$ 3,063,473	$ 411,782	$ (8,108)
Shares, Issued beginning balance at Dec. 31, 2009		30,635
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from sale of capital stock, shares		651
Proceeds from sale of capital stock, par value	65,124	65,124
Net shares reclassified to mandatorily redeemable capital stock, shares		(76)
Net shares reclassified to mandatorily redeemable capital stock, par value	(7,561)	(7,561)
Comprehensive income:
Net Income	83,761		83,761
Other comprehensive income:
Net unrealized losses on available-for-sale securities	(178)			(178)
Pension and postretirement benefits	451			451
Total comprehensive income	84,034
Dividends on capital stock:
Cash	(72,191)		(72,191)
Ending balance at Jun. 30, 2010	3,536,553	3,121,036	423,352	(7,835)
Shares, Issued ending balance at Jun. 30, 2010		31,210
Beginning balance at Dec. 31, 2010	3,522,528	3,092,377	437,874	(7,723)
Shares, Issued beginning balance at Dec. 31, 2010		30,924
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from sale of capital stock, shares		207
Proceeds from sale of capital stock, par value	20,698	20,698
Net shares reclassified to mandatorily redeemable capital stock, shares		0
Net shares reclassified to mandatorily redeemable capital stock, par value	(12)	(12)
Comprehensive income:
Net Income	79,858		79,858
Other comprehensive income:
Net unrealized losses on available-for-sale securities	(89)			(89)
Pension and postretirement benefits	433			433
Total comprehensive income	80,202
Dividends on capital stock:
Cash	(69,518)		(69,518)
Ending balance at Jun. 30, 2011	$ 3,553,898	$ 3,113,063	$ 448,214	$ (7,379)
Shares, Issued ending balance at Jun. 30, 2011		31,131