The following table summarizes fair value measurements by level at December 31, 2014, for assets and liabilities measured at fair value on a recurring basis ($ in millions):



	Level I		Level II		Level III		Total
Assets
Cash and cash equivalents	$	1,546	$	—	$	—	$	1,546
Investments available for sale:
U.S. Treasury securities and obligations of U.S. government corporations and agencies	$	360	$	17	$	—	$	377
Corporate securities	—		556		—		556
Municipal securities:
General obligation	—		54		—		54
Pre-refunded	—		5		—		5
Revenue	—		102		—		102
Variable rate demand notes	—		14		—		14
Asset backed securities	—		180		—		180
Mortgage backed securities	—		85		—		85
Total investments	$	360	$	1,013	$	—	$	1,373
Restricted deposits available for sale:
Cash and cash equivalents	$	79	$	—	$	—	$	79
Certificates of deposit	6		—		—		6
U.S. Treasury securities and obligations of U.S. government corporations and agencies	15		—		—		15
Total restricted deposits	$	100	$	—	$	—	$	100
Other long term assets:
Interest rate swap agreements	$	—	$	11	$	—	$	11
Total assets at fair value	$	2,006	$	1,024	$	—	$	3,030

The following table summarizes fair value measurements by level at December 31, 2013, for assets and liabilities measured at fair value on a recurring basis ($ in millions):



	Level I		Level II		Level III		Total
Assets
Cash and cash equivalents	$	974	$	—	$	—	$	974
Investments available for sale:
U.S. Treasury securities and obligations of U.S. government corporations and agencies	$	213	$	12	$	—	$	225
Corporate securities	—		296		—		296
Municipal securities:
General obligation	—		54		—		54
Pre-refunded	—		11		—		11
Revenue	—		69		—		69
Variable rate demand notes	—		29		—		29
Asset backed securities	—		139		—		139
Mortgage backed securities	—		34		—		34
Total investments	$	213	$	644	$	—	$	857
Restricted deposits available for sale:
Cash and cash equivalents	$	27	$	—	$	—	$	27
Certificates of deposit	6		—		—		6
U.S. Treasury securities and obligations of U.S. government corporations and agencies	14		—		—		14
Total restricted deposits	$	47	$	—	$	—	$	47
Other long term assets:
Interest rate swap agreements	$	—	$	10	$	—	$	10
Total assets at fair value	$	1,234	$	654	$	—	$	1,888