Postretirement Benefit Plans - Trust Assets Measured at Fair Value (Details) - Postretirement Benefit Plans [Member] - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assets:
Total Assets	$ 112.1	$ 120.7	$ 240.1
Fair Value, Measurements, Recurring [Member]
Assets:
Total Assets	112.1	120.7
Fair Value, Measurements, Recurring [Member] | Cash and cash equivalents [Member]
Assets:
Total Assets	3.0	7.9
Fair Value, Measurements, Recurring [Member] | Common collective funds - equity [Member]
Assets:
Total Assets	28.2	39.9
Fair Value, Measurements, Recurring [Member] | International Equity Funds [Member]
Assets:
Total Assets	18.3	22.1
Fair Value, Measurements, Recurring [Member] | Common collective funds - fixed income [Member]
Assets:
Total Assets	62.6	50.8
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Total Assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Cash and cash equivalents [Member]
Assets:
Total Assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Common collective funds - equity [Member]
Assets:
Total Assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | International Equity Funds [Member]
Assets:
Total Assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Common collective funds - fixed income [Member]
Assets:
Total Assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Total Assets	112.1	120.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Cash and cash equivalents [Member]
Assets:
Total Assets	3.0	7.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Common collective funds - equity [Member]
Assets:
Total Assets	28.2	39.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | International Equity Funds [Member]
Assets:
Total Assets	18.3	22.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Common collective funds - fixed income [Member]
Assets:
Total Assets	62.6	50.8
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets:
Total Assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Cash and cash equivalents [Member]
Assets:
Total Assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Common collective funds - equity [Member]
Assets:
Total Assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | International Equity Funds [Member]
Assets:
Total Assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Common collective funds - fixed income [Member]
Assets:
Total Assets	$ 0.0	$ 0.0