Subsequent Events: (Details) (Subsequent event, USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended
	Jul. 30, 2013	Jul. 29, 2013 Ten-year fixed rate bonds with 3.375 percent coupon rate	Jul. 29, 2013 Two-year floating rate bonds
Subsequent events:
Dividend declared (in dollars per share)	$ 0.95
Dividend declared, date	Jul. 30, 2013
Dividend payable, date	Sep. 10, 2013
Shareholders of record, date	Aug. 09, 2013
Bond offering date		Jul. 29, 2013	Jul. 29, 2013
Bond offered and priced		$ 1,500	$ 650
Maturity term		10 years	2 years
Debt instrument, interest rate (as a percent)		3.75%
Reference rate			three-month LIBOR
Interest rate margin (as a percent)			0.03%
Bond issuance date		Aug. 01, 2013	Aug. 01, 2013