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Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
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Investment [Line Items] |  |
Within one year, Amortized Cost | $ 113,488 |
After one year through five years, Amortized Cost | 239,333 |
After five years through ten years, Amortized Cost | 98,935 |
After ten years, Amortized Cost | 134,328 |
Held-to-maturity, Amortized Cost, Totals | 598,658 | $ 562,434
Within one year, Fair Value | 113,988 |
After one year through five years, Fair Value | 243,851 |
After five years through ten years, Fair Value | 101,450 |
After ten years, Fair Value | 137,424 |
Held-to-maturity, Fair Value, Totals | 610,151 | $ 568,931
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost | 10,928 |
Held-to-maturity, Fair Value | 11,676 |
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost | 1,646 |
Held-to-maturity, Fair Value | $ 1,762 |

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- Definition Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 4: 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) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.6) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Fair value of held-to-maturity debt securities maturing after the tenth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition Fair value of held-to-maturity debt securities maturing in the sixth through tenth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition Fair value of held-to-maturity debt securities maturing in the second through fifth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition Amount of held-to-maturity debt securities at fair value that do not have a single maturity date and which the entity has decided to disclose separately rather than allocating the fair value over several maturity groupings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 + Details
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- Definition Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, over several maturity groupings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (d),(f) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 + Details
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