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Investments - Securities and Investments, Recorded at Either Fair Value or Cost (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 114,139 | $ 190,146
Gross Unrealized Gains | 5,262 | 2,709
Gross Unrealized Losses | (1,363) | (2,132)
Available-for-sale-securities, Fair Value/Carrying Value | 118,038 | 190,723
Restricted investment held at cost | 27,800 | 27,800
Other investments held at cost | 547 | 767
Total carrying value of available-for-sale investments | 146,385 | 219,290
Deferred Compensation Plan Assets [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 17,804 | 12,367
Gross Unrealized Gains | 2,175 | 2,271
Gross Unrealized Losses | (96) | (42)
Available-for-sale-securities, Fair Value/Carrying Value | 19,883 | 14,596
Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 32,654 | 66,522
Gross Unrealized Gains | 44 | 64
Gross Unrealized Losses | (155) | (174)
Available-for-sale-securities, Fair Value/Carrying Value | 32,543 | 66,412
Municipal Fixed-Rate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 2,902 | 11,799
Gross Unrealized Gains | 2 | 12
Gross Unrealized Losses | (22) | (37)
Available-for-sale-securities, Fair Value/Carrying Value | 2,882 | 11,774
Asset-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 6,545 | 10,201
Gross Unrealized Gains | 1 | 19
Gross Unrealized Losses | (20) | (14)
Available-for-sale-securities, Fair Value/Carrying Value | 6,526 | 10,206
Mortgage/Agency-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 5,554 | 13,080
Gross Unrealized Gains | 1 | 15
Gross Unrealized Losses | (46) | (91)
Available-for-sale-securities, Fair Value/Carrying Value | 5,509 | 13,004
U.S. Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 14,477 | 30,022
Gross Unrealized Gains |  | 15
Gross Unrealized Losses | (174) | (270)
Available-for-sale-securities, Fair Value/Carrying Value | 14,303 | 29,767
Foreign Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 725 | 3,729
Gross Unrealized Gains | 5 | 2
Gross Unrealized Losses |  | (1)
Available-for-sale-securities, Fair Value/Carrying Value | 730 | 3,730
Variable Rate Demand Notes [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost |  | 11,855
Available-for-sale-securities, Fair Value/Carrying Value |  | 11,855
Marketable Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 33,478 | 30,571
Gross Unrealized Gains | 3,034 | 311
Gross Unrealized Losses | (850) | (1,503)
Available-for-sale-securities, Fair Value/Carrying Value | $ 35,662 | $ 29,379

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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 Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and 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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