Investments - Schedule of Unrealized Investment Losses by Class of Investment (Details) $ in Thousands	Sep. 30, 2017 USD ($)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	$ 726,647
Fair Value, Twelve Months or Longer	695,383
Fair Value, Total	1,422,030
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(10,282)
Unrealized Loss, Twelve Months or Longer	(82,356)
Unrealized Loss Total	(92,638)
Investment grade [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	726,378
Fair Value, Twelve Months or Longer	453,082
Fair Value, Total	1,179,460
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(10,246)
Unrealized Loss, Twelve Months or Longer	(21,375)
Unrealized Loss Total	(31,621)
Investment grade [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	97,370
Fair Value, Twelve Months or Longer	5,422
Fair Value, Total	102,792
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(610)
Unrealized Loss, Twelve Months or Longer	(488)
Unrealized Loss Total	(1,098)
Investment grade [Member] | States, municipalities, and political subdivisions [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	10,115
Fair Value, Twelve Months or Longer	691
Fair Value, Total	10,806
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(70)
Unrealized Loss, Twelve Months or Longer	(2)
Unrealized Loss Total	(72)
Investment grade [Member] | Corporates [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	602,963
Fair Value, Twelve Months or Longer	446,969
Fair Value, Total	1,049,932
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(9,433)
Unrealized Loss, Twelve Months or Longer	(20,885)
Unrealized Loss Total	(30,318)
Investment grade [Member] | Corporates [Member] | Financial [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	94,944
Fair Value, Twelve Months or Longer	65,729
Fair Value, Total	160,673
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(1,024)
Unrealized Loss, Twelve Months or Longer	(2,271)
Unrealized Loss Total	(3,295)
Investment grade [Member] | Corporates [Member] | Utilities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	101,687
Fair Value, Twelve Months or Longer	40,075
Fair Value, Total	141,762
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(1,643)
Unrealized Loss, Twelve Months or Longer	(1,012)
Unrealized Loss Total	(2,655)
Investment grade [Member] | Corporates [Member] | Energy [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	37,009
Fair Value, Twelve Months or Longer	93,841
Fair Value, Total	130,850
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(512)
Unrealized Loss, Twelve Months or Longer	(6,555)
Unrealized Loss Total	(7,067)
Investment grade [Member] | Corporates [Member] | Other corporate sectors [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	369,323
Fair Value, Twelve Months or Longer	247,324
Fair Value, Total	616,647
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(6,254)
Unrealized Loss, Twelve Months or Longer	(11,047)
Unrealized Loss Total	(17,301)
Investment grade [Member] | Other asset-backed securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	9,983
Fair Value, Twelve Months or Longer	0
Fair Value, Total	9,983
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(17)
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss Total	(17)
Investment grade [Member] | Redeemable preferred stocks [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	5,947
Fair Value, Twelve Months or Longer	0
Fair Value, Total	5,947
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(116)
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss Total	(116)
Investment grade [Member] | Redeemable preferred stocks [Member] | Utilities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	5,947
Fair Value, Twelve Months or Longer	0
Fair Value, Total	5,947
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(116)
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss Total	(116)
Below investment grade [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	269
Fair Value, Twelve Months or Longer	242,301
Fair Value, Total	242,570
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(36)
Unrealized Loss, Twelve Months or Longer	(60,981)
Unrealized Loss Total	(61,017)
Below investment grade [Member] | States, municipalities, and political subdivisions [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	269
Fair Value, Twelve Months or Longer	0
Fair Value, Total	269
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(36)
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss Total	(36)
Below investment grade [Member] | Corporates [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	207,215
Fair Value, Total	207,215
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	(48,957)
Unrealized Loss Total	(48,957)
Below investment grade [Member] | Corporates [Member] | Financial [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	83,164
Fair Value, Total	83,164
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	(22,585)
Unrealized Loss Total	(22,585)
Below investment grade [Member] | Corporates [Member] | Energy [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	78,721
Fair Value, Total	78,721
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	(21,003)
Unrealized Loss Total	(21,003)
Below investment grade [Member] | Corporates [Member] | Other corporate sectors [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	45,330
Fair Value, Total	45,330
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	(5,369)
Unrealized Loss Total	(5,369)
Below investment grade [Member] | Collateralized debt obligations [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	11,006
Fair Value, Total	11,006
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	(8,994)
Unrealized Loss Total	(8,994)
Below investment grade [Member] | Redeemable preferred stocks [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	24,080
Fair Value, Total	24,080
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	(3,030)
Unrealized Loss Total	(3,030)
Below investment grade [Member] | Redeemable preferred stocks [Member] | Financial [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	0
Fair Value, Twelve Months or Longer	24,080
Fair Value, Total	24,080
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	(3,030)
Unrealized Loss Total	$ (3,030)