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Pension Benefit Obligations - Level 3 (Details) $ in Millions	12 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) plan
Pension Plan, Defined Benefit
Post-retirement plan
Fair values of plan assets	$ 1,384.8	$ 1,384.8	$ 1,430.1
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	1,384.8
Fair value of plan assets at end of year	1,430.1	1,384.8
Pension Plan, Defined Benefit | Cash and Cash Equivalents
Post-retirement plan
Fair values of plan assets	44.4	44.4	70.6
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	44.4
Fair value of plan assets at end of year	70.6	44.4
Pension Plan, Defined Benefit | Global equity securities
Post-retirement plan
Fair values of plan assets	52.8	52.8	49.4
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	52.8
Fair value of plan assets at end of year	49.4	52.8
Pension Plan, Defined Benefit | Domestic equity securities
Post-retirement plan
Fair values of plan assets	60.0	60.0	57.1
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	60.0
Fair value of plan assets at end of year	57.1	60.0
Pension Plan, Defined Benefit | Diversified Funds
Post-retirement plan
Fair values of plan assets	287.4	287.4	223.1
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	287.4
Fair value of plan assets at end of year	223.1	287.4
Pension Plan, Defined Benefit | Equity Funds
Post-retirement plan
Fair values of plan assets	309.6	309.6	362.0
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	309.6
Fair value of plan assets at end of year	362.0	309.6
Pension Plan, Defined Benefit | Fixed Income Funds
Post-retirement plan
Fair values of plan assets	542.5	542.5	548.5
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	542.5
Fair value of plan assets at end of year	548.5	542.5
Pension Plan, Defined Benefit | Hedge Funds
Post-retirement plan
Fair values of plan assets	53.0	53.0	62.5
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	53.0
Fair value of plan assets at end of year	62.5	53.0
Pension Plan, Defined Benefit | Assets Held by Insurance Company
Post-retirement plan
Fair values of plan assets	35.1	35.1	32.3
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	35.1
Fair value of plan assets at end of year	32.3	35.1
Pension Plan, Defined Benefit | Other
Post-retirement plan
Fair values of plan assets	24.6		24.6
Changes in the fair value of Level 3 assets
Fair value of plan assets at end of year	24.6
Pension Plan, Defined Benefit | Level 1
Post-retirement plan
Fair values of plan assets	11.0	11.0	23.1
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	11.0
Fair value of plan assets at end of year	23.1	11.0
Pension Plan, Defined Benefit | Level 1 | Cash and Cash Equivalents
Post-retirement plan
Fair values of plan assets	11.0	11.0	14.3
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	11.0
Fair value of plan assets at end of year	14.3	11.0
Pension Plan, Defined Benefit | Level 1 | Equity Funds
Post-retirement plan
Fair values of plan assets	5.1		5.1
Changes in the fair value of Level 3 assets
Fair value of plan assets at end of year	5.1
Pension Plan, Defined Benefit | Level 1 | Fixed Income Funds
Post-retirement plan
Fair values of plan assets	3.7		3.7
Changes in the fair value of Level 3 assets
Fair value of plan assets at end of year	3.7
Pension Plan, Defined Benefit | Level 2
Post-retirement plan
Fair values of plan assets	1,360.2	1,360.2	1,392.9
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	1,360.2
Fair value of plan assets at end of year	1,392.9	1,360.2
Pension Plan, Defined Benefit | Level 2 | Cash and Cash Equivalents
Post-retirement plan
Fair values of plan assets	33.4	33.4	56.3
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	33.4
Fair value of plan assets at end of year	56.3	33.4
Pension Plan, Defined Benefit | Level 2 | Global equity securities
Post-retirement plan
Fair values of plan assets	52.8	52.8	49.4
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	52.8
Fair value of plan assets at end of year	49.4	52.8
Pension Plan, Defined Benefit | Level 2 | Domestic equity securities
Post-retirement plan
Fair values of plan assets	60.0	60.0	57.1
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	60.0
Fair value of plan assets at end of year	57.1	60.0
Pension Plan, Defined Benefit | Level 2 | Diversified Funds
Post-retirement plan
Fair values of plan assets	287.4	287.4	223.1
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	287.4
Fair value of plan assets at end of year	223.1	287.4
Pension Plan, Defined Benefit | Level 2 | Equity Funds
Post-retirement plan
Fair values of plan assets	309.6	309.6	356.9
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	309.6
Fair value of plan assets at end of year	356.9	309.6
Pension Plan, Defined Benefit | Level 2 | Fixed Income Funds
Post-retirement plan
Fair values of plan assets	542.5	542.5	544.8
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	542.5
Fair value of plan assets at end of year	544.8	542.5
Pension Plan, Defined Benefit | Level 2 | Hedge Funds
Post-retirement plan
Fair values of plan assets	39.4	39.4	48.4
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	39.4
Fair value of plan assets at end of year	48.4	39.4
Pension Plan, Defined Benefit | Level 2 | Assets Held by Insurance Company
Post-retirement plan
Fair values of plan assets	35.1	35.1	32.3
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	35.1
Fair value of plan assets at end of year	32.3	35.1
Pension Plan, Defined Benefit | Level 2 | Other
Post-retirement plan
Fair values of plan assets	24.6		24.6
Changes in the fair value of Level 3 assets
Fair value of plan assets at end of year	24.6
Pension Plan, Defined Benefit | Level 3
Post-retirement plan
Fair values of plan assets	13.6	13.6	14.1
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	13.6
Fair value of plan assets at end of year	14.1	13.6
Pension Plan, Defined Benefit | Level 3 | Hedge Funds
Post-retirement plan
Fair values of plan assets	13.6	13.7	$ 14.1
Changes in the fair value of Level 3 assets
Fair value of plan assets at beginning of year	13.6	13.7
Actual return on plan assets, relating to assets still held at reporting date	0.5	(0.1)
Fair value of plan assets at end of year	14.1	13.6
Multiemployer Pension Plans
Multiemployer Pension Plans
Number of minimum construction-industry pension plans participated | plan			200
Aggregate contributions to plans	$ 49.5	$ 54.5

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