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Marketable Investment Securities - Schedule of Fair Value for Available-for-Sale Securities by Major Security Type and Class of Security (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Jun. 30, 2017 | Dec. 31, 2016 | Jun. 30, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cash and cash equivalents, Amortized cost | $ 88.7 | $ 102.4 | $ 108.0 | $ 68.5
Available-for-sale, Amortized cost | 202.4 | 199.3 |  |
Gross unrealized holding gains |  | 0.1 |  |
Gross unrealized holding losses | (0.4) | (0.2) |  |
Available-for-sale, Estimated fair value | 202.0 | 199.2 |  |
Amortized cost | 202.4 | 199.3 |  |
Estimated fair value | 202.0 | 199.2 |  |
Cash [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cash and cash equivalents, Amortized cost | 85.9 | 83.5 |  |
Cash and cash equivalents, Estimated fair value | 85.9 | 83.5 |  |
Cash Equivalents [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cash and cash equivalents, Amortized cost | 2.8 | 18.9 |  |
Cash and cash equivalents, Estimated fair value | 2.8 | 18.9 |  |
Total Cash and Cash Equivalents [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cash and cash equivalents, Amortized cost | 88.7 | 102.4 |  |
Cash and cash equivalents, Estimated fair value | 88.7 | 102.4 |  |
Corporate Bonds and Notes [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Available-for-sale, Amortized cost | 57.9 | 45.4 |  |
Gross unrealized holding gains |  | 0.1 |  |
Gross unrealized holding losses | (0.2) | (0.1) |  |
Available-for-sale, Estimated fair value | 57.7 | 45.4 |  |
Municipal Bonds [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Available-for-sale, Amortized cost | 34.9 | 32.7 |  |
Gross unrealized holding losses | (0.1) |  |  |
Available-for-sale, Estimated fair value | 34.8 | 32.7 |  |
Federal Agency Issues [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Available-for-sale, Amortized cost | 12.6 | 11.6 |  |
Gross unrealized holding losses | (0.1) | (0.1) |  |
Available-for-sale, Estimated fair value | 12.5 | 11.5 |  |
US Government Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Available-for-sale, Amortized cost | 8.3 | 7.2 |  |
Available-for-sale, Estimated fair value | $ 8.3 | $ 7.2 |  |

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3044-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Sum of investments and unrestricted cash as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1,2) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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