Condensed Consolidating Statements of Cash Flows (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating activities:
Net cash provided (used)	$ 906.0	$ 798.3	$ 673.2
Investing activities:
Capital expenditures	(668.2)	(594.8)	(517.1)
Purchase or replacement of equipment under operating leases	(302.1)	(211.8)	(22.9)
Property investments in MSLLC	(26.7)	(31.6)	(35.2)
Proceeds from repayment of loans to affiliates		0	0
Loans to affiliates		0
Contribution to consolidated affiliates	0
Other investing activities	14.1	4.9	23.3
Net cash used for investing activities	(982.9)	(833.3)	(551.9)
Financing activities:
Proceeds from short-term borrowings	15,368.8	0	0
Repayment of short-term borrowings	(14,920.2)	0	0
Proceeds from issuance of long-term debt	175.0	1,918.4	329.6
Repayment of long-term debt	(508.0)	(1,343.2)	(375.9)
Debt costs	(4.9)	(117.8)	(22.1)
Dividends paid	(116.6)	(71.2)	(86.1)
Proceeds from loans from affiliates		0
Repayment of loans from affiliates		0	0
Contributions from affiliates	0
Other financing activities	(3.6)	5.7	33.4
Net cash provided by (used for) financing activities	(4.6)	391.9	(121.1)
Cash and cash equivalents:
Net increase (decrease) during each year	(81.5)	356.9	0.2
At beginning of year	429.5	72.6	72.4
At end of year	348.0	429.5	72.6
Parent [Member]
Operating activities:
Net cash provided (used)	415.7	70.7	53.3
Investing activities:
Capital expenditures	0	0	0
Purchase or replacement of equipment under operating leases	0	0	0
Property investments in MSLLC	0	0	0
Proceeds from repayment of loans to affiliates		0	0
Loans to affiliates		0
Contribution to consolidated affiliates	(299.6)
Other investing activities	(1.0)	(4.9)	(0.5)
Net cash used for investing activities	(300.6)	(4.9)	(0.5)
Financing activities:
Proceeds from short-term borrowings	0
Repayment of short-term borrowings	0
Proceeds from issuance of long-term debt	0	0	0
Repayment of long-term debt	0	0	0
Debt costs		0	0
Dividends paid	(116.6)	(71.2)	(86.1)
Proceeds from loans from affiliates		0
Repayment of loans from affiliates		0	0
Contributions from affiliates	0
Other financing activities	1.3	5.7	33.4
Net cash provided by (used for) financing activities	(115.3)	(65.5)	(52.7)
Cash and cash equivalents:
Net increase (decrease) during each year	(0.2)	0.3	0.1
At beginning of year	0.4	0.1	0
At end of year	0.2	0.4	0.1
KCSR [Member]
Operating activities:
Net cash provided (used)	306.6	272.2	236.4
Investing activities:
Capital expenditures	(477.8)	(417.4)	(281.2)
Purchase or replacement of equipment under operating leases	(203.6)	(130.9)	(19.7)
Property investments in MSLLC	0	0	0
Proceeds from repayment of loans to affiliates		181.4	87.2
Loans to affiliates		(69.5)
Contribution to consolidated affiliates	0
Other investing activities	9.7	(5.1)	5.8
Net cash used for investing activities	(671.7)	(441.5)	(207.9)
Financing activities:
Proceeds from short-term borrowings	15,068.8
Repayment of short-term borrowings	(14,920.2)
Proceeds from issuance of long-term debt	175.0	687.6	329.6
Repayment of long-term debt	(423.5)	(344.3)	(356.2)
Debt costs		(7.5)	(21.3)
Dividends paid	0	0	0
Proceeds from loans from affiliates		0
Repayment of loans from affiliates		0	0
Contributions from affiliates	299.3
Other financing activities	(1.4)	0	0
Net cash provided by (used for) financing activities	198.0	335.8	(47.9)
Cash and cash equivalents:
Net increase (decrease) during each year	(167.1)	166.5	(19.4)
At beginning of year	196.1	29.6	49.0
At end of year	29.0	196.1	29.6
Guarantor Subsidiaries [Member]
Operating activities:
Net cash provided (used)	2.1	2.6	2.3
Investing activities:
Capital expenditures	(1.7)	(2.0)	(2.1)
Purchase or replacement of equipment under operating leases	0	0	0
Property investments in MSLLC	0	0	0
Proceeds from repayment of loans to affiliates		0	0
Loans to affiliates		0
Contribution to consolidated affiliates	0
Other investing activities	(1.1)	(1.1)	(0.5)
Net cash used for investing activities	(2.8)	(3.1)	(2.6)
Financing activities:
Proceeds from short-term borrowings	0
Repayment of short-term borrowings	0
Proceeds from issuance of long-term debt	0	0	0
Repayment of long-term debt	(0.1)	(0.1)	(0.1)
Debt costs		0	0
Dividends paid	0	0	0
Proceeds from loans from affiliates		0
Repayment of loans from affiliates		0	0
Contributions from affiliates	1.1
Other financing activities	0	0.7	0.5
Net cash provided by (used for) financing activities	1.0	0.6	0.4
Cash and cash equivalents:
Net increase (decrease) during each year	0.3	0.1	0.1
At beginning of year	0.2	0.1	0
At end of year	0.5	0.2	0.1
Non-Guarantor Subsidiaries [Member]
Operating activities:
Net cash provided (used)	495.9	463.8	389.2
Investing activities:
Capital expenditures	(190.2)	(175.8)	(233.8)
Purchase or replacement of equipment under operating leases	(98.5)	(80.9)	(3.2)
Property investments in MSLLC	(26.7)	(31.6)	(35.2)
Proceeds from repayment of loans to affiliates		0	0
Loans to affiliates		0
Contribution to consolidated affiliates	0
Other investing activities	5.8	10.8	17.5
Net cash used for investing activities	(309.6)	(277.5)	(254.7)
Financing activities:
Proceeds from short-term borrowings	300.0
Repayment of short-term borrowings	0
Proceeds from issuance of long-term debt	0	1,230.8	0
Repayment of long-term debt	(84.4)	(998.8)	(19.6)
Debt costs		(110.3)	(0.8)
Dividends paid	(314.3)	(11.0)	(8.0)
Proceeds from loans from affiliates		69.5
Repayment of loans from affiliates		(181.4)	(87.2)
Contributions from affiliates	1.4
Other financing activities	(3.5)	4.9	0.5
Net cash provided by (used for) financing activities	(100.8)	3.7	(115.1)
Cash and cash equivalents:
Net increase (decrease) during each year	85.5	190.0	19.4
At beginning of year	232.8	42.8	23.4
At end of year	318.3	232.8	42.8
Consolidating Adjustments [Member]
Operating activities:
Net cash provided (used)	(314.3)	(11.0)	(8.0)
Investing activities:
Capital expenditures	1.5	0.4	0
Purchase or replacement of equipment under operating leases	0	0	0
Property investments in MSLLC	0	0	0
Proceeds from repayment of loans to affiliates		(181.4)	(87.2)
Loans to affiliates		69.5
Contribution to consolidated affiliates	299.6
Other investing activities	0.7	5.2	1.0
Net cash used for investing activities	301.8	(106.3)	(86.2)
Financing activities:
Proceeds from short-term borrowings	0
Repayment of short-term borrowings	0
Proceeds from issuance of long-term debt	0	0	0
Repayment of long-term debt	0	0	0
Debt costs		0	0
Dividends paid	314.3	11.0	8.0
Proceeds from loans from affiliates		(69.5)
Repayment of loans from affiliates		181.4	87.2
Contributions from affiliates	(301.8)
Other financing activities	0	(5.6)	(1.0)
Net cash provided by (used for) financing activities	12.5	117.3	94.2
Cash and cash equivalents:
Net increase (decrease) during each year	0	0	0
At beginning of year	0	0	0
At end of year	$ 0	$ 0	$ 0