Statement of Equity (USD $) In Thousands, unless otherwise specified	Total	Common Stock [Member]	Retained Earnings, Unrestricted [Member]	Retained Earnings, Restricted [Member]	Retained Earnings, Total [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balance at Dec. 31, 2011	$ 3,559,107	$ 3,125,895	$ 432,530	$ 11,683	$ 444,213	$ (11,001)
Shares, Issued beginning balance at Dec. 31, 2011		31,259
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from sale of capital stock, shares		307
Proceeds from sale of capital stock, par value	30,712	30,712
Net shares reclassified to mandatorily redeemable capital stock, shares		(157)
Net shares reclassified to mandatorily redeemable capital stock, value	(15,704)	(15,704)
Comprehensive Income	59,093		46,155	11,538	57,693	1,400
Dividends on capital stock:
Cash	(35,316)		(35,316)		(35,316)
Ending balance at Mar. 31, 2012	3,597,892	3,140,903	443,369	23,221	466,590	(9,601)
Shares, Issued ending balance at Mar. 31, 2012		31,409
Beginning balance at Dec. 31, 2012	4,536,769	4,010,622	479,253	58,628	537,881	(11,734)
Shares, Issued beginning balance at Dec. 31, 2012		40,106
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from sale of capital stock, shares		4,742
Proceeds from sale of capital stock, par value	474,153	474,153
Net shares reclassified to mandatorily redeemable capital stock, shares		(182)
Net shares reclassified to mandatorily redeemable capital stock, value	(18,225)	(18,225)
Comprehensive Income	64,004		50,808	12,702	63,510	494
Dividends on capital stock:
Cash	(38,454)		(38,454)		(38,454)
Ending balance at Mar. 31, 2013	$ 5,018,247	$ 4,466,550	$ 491,607	$ 71,330	$ 562,937	$ (11,240)
Shares, Issued ending balance at Mar. 31, 2013		44,666