The following table summarizes fair value measurements by level at December 31, 2017, 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	$	4,072	$	—	$	—	$	4,072
Investments available for sale:
U.S. Treasury securities and obligations of U.S. government corporations and agencies	$	195	$	—	$	—	$	195
Corporate securities	—		2,210		—		2,210
Municipal securities	—		2,087		—		2,087
Asset-backed securities	—		437		—		437
Residential mortgage-backed securities	—		332		—		332
Commercial mortgage-backed securities	—		271		—		271
Total investments	$	195	$	5,337	$	—	$	5,532
Restricted deposits available for sale:
Cash and cash equivalents	$	17	$	—	$	—	$	17
Certificates of deposit	4		—		—		4
U.S. Treasury securities and obligations of U.S. government corporations and agencies	114		—		—		114
Total restricted deposits	$	135	$	—	$	—	$	135
Other long-term assets:
Interest rate swap agreements	$	—	$	1	$	—	$	1
Total assets at fair value	$	4,402	$	5,338	$	—	$	9,740

Liabilities
Other long-term liabilities:
Interest rate swap agreements	$	—	$	72	$	—	$	72
Total liabilities at fair value	$	—	$	72	$	—	$	72

The following table summarizes fair value measurements by level at December 31, 2016, 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	$	3,930	$	—	$	—	$	3,930
Investments available for sale:
U.S. Treasury securities and obligations of U.S. government corporations and agencies	$	221	$	15	$	—	$	236
Corporate securities	—		1,932		—		1,932
Municipal securities	—		1,733		—		1,733
Asset-backed securities	—		317		—		317
Residential mortgage-backed securities	—		215		—		215
Commercial mortgage-backed securities	—		338		—		338
Total investments	$	221	$	4,550	$	—	$	4,771
Restricted deposits available for sale:
Cash and cash equivalents	$	6	$	—	$	—	$	6
Certificates of deposit	5		—		—		5
U.S. Treasury securities and obligations of U.S. government corporations and agencies	127		—		—		127
Total restricted deposits	$	138	$	—	$	—	$	138
Other long-term assets:
Interest rate swap agreements	$	—	$	4	$	—	$	4
Total assets at fair value	$	4,289	$	4,554	$	—	$	8,843

Liabilities
Other long-term liabilities:
Interest rate swap agreements	$	—	$	62	$	—	$	62
Total liabilities at fair value	$	—	$	62	$	—	$	62