Significant Accounting Policies (Income Statement) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues	$ 1,452	$ 1,425	$ 1,422	$ 1,340	$ 1,330	$ 1,278	$ 1,267	$ 1,038	$ 5,639	$ 4,913	$ 4,171
Cost of revenues									(4,986)	(4,329)	(3,732)
Equity in earnings of unconsolidated affiliates	11	9	15	0	29	17	12	21	35	79	70
Income before income taxes and noncontrolling interests									268	396	247
(Provision) benefit for income taxes									(59)	(86)	193
Net income	$ 59	$ 58	$ 50	$ 42	$ 53	$ 56	$ 64	$ 137	$ 209	$ 310	$ 440
Net income attributable to KBR per share:
Basic (usd per share)	$ 0.41	$ 0.39	$ 0.34	$ 0.28	$ 0.33	$ 0.38	$ 0.31	$ 0.96	$ 1.42	$ 1.99	$ 3.05
Diluted (usd per share)	$ 0.40	$ 0.39	$ 0.34	$ 0.28	$ 0.33	$ 0.38	$ 0.31	$ 0.96	$ 1.41	$ 1.99	$ 3.05
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										$ 4,904
Cost of revenues										(4,328)
Equity in earnings of unconsolidated affiliates										75
Income before income taxes and noncontrolling interests										384
(Provision) benefit for income taxes										(85)
Net income										$ 300
Net income attributable to KBR per share:
Basic (usd per share)										$ 1.92
Diluted (usd per share)										$ 1.91
Adjustments Due to Topic 606 | Difference between Revenue Guidance in Effect before and after Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										$ 9
Cost of revenues										1
Equity in earnings of unconsolidated affiliates										4
Income before income taxes and noncontrolling interests										12
(Provision) benefit for income taxes										1
Net income										$ 10
Net income attributable to KBR per share:
Basic (usd per share)										$ 0.07
Diluted (usd per share)										$ 0.08