Investments - Schedule of Unrealized Investment Losses by Class of Investment (Details) $ in Thousands	Jun. 30, 2017 USD ($)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	$ 882,317
Fair Value, Twelve Months or Longer	529,336
Fair Value, Total	1,411,653
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(19,828)
Unrealized Loss, Twelve Months or Longer	(83,069)
Unrealized Loss Total	(102,897)
Investment grade [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	862,144
Fair Value, Twelve Months or Longer	275,511
Fair Value, Total	1,137,655
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(19,643)
Unrealized Loss, Twelve Months or Longer	(14,778)
Unrealized Loss Total	(34,421)
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	114,030
Fair Value, Twelve Months or Longer	1,501
Fair Value, Total	115,531
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(1,194)
Unrealized Loss, Twelve Months or Longer	(502)
Unrealized Loss Total	(1,696)
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	13,502
Fair Value, Twelve Months or Longer	666
Fair Value, Total	14,168
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(155)
Unrealized Loss, Twelve Months or Longer	(30)
Unrealized Loss Total	(185)
Investment grade [Member] | Foreign governments [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	0
Fair Value, Total	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss Total	0
Investment grade [Member] | Corporates [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	718,830
Fair Value, Twelve Months or Longer	273,344
Fair Value, Total	992,174
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(18,059)
Unrealized Loss, Twelve Months or Longer	(14,246)
Unrealized Loss Total	(32,305)
Investment grade [Member] | Corporates [Member] | Financial [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	123,632
Fair Value, Twelve Months or Longer	47,847
Fair Value, Total	171,479
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(3,735)
Unrealized Loss, Twelve Months or Longer	(1,157)
Unrealized Loss Total	(4,892)
Investment grade [Member] | Corporates [Member] | Utilities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	120,890
Fair Value, Twelve Months or Longer	17,710
Fair Value, Total	138,600
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(2,481)
Unrealized Loss, Twelve Months or Longer	(870)
Unrealized Loss Total	(3,351)
Investment grade [Member] | Corporates [Member] | Energy [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	36,740
Fair Value, Twelve Months or Longer	137,991
Fair Value, Total	174,731
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(486)
Unrealized Loss, Twelve Months or Longer	(9,595)
Unrealized Loss Total	(10,081)
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	437,568
Fair Value, Twelve Months or Longer	69,796
Fair Value, Total	507,364
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(11,357)
Unrealized Loss, Twelve Months or Longer	(2,624)
Unrealized Loss Total	(13,981)
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	9,927
Fair Value, Twelve Months or Longer	0
Fair Value, Total	9,927
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(15)
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss Total	(15)
Investment grade [Member] | Redeemable preferred stocks [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	5,855
Fair Value, Twelve Months or Longer	0
Fair Value, Total	5,855
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(220)
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss Total	(220)
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	0
Fair Value, Total	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss Total	0
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	5,855
Fair Value, Twelve Months or Longer	0
Fair Value, Total	5,855
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(220)
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss Total	(220)
Below investment grade [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	20,173
Fair Value, Twelve Months or Longer	253,825
Fair Value, Total	273,998
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(185)
Unrealized Loss, Twelve Months or Longer	(68,291)
Unrealized Loss Total	(68,476)
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	0
Fair Value, Twelve Months or Longer	0
Fair Value, Total	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	0
Unrealized Loss Total	0
Below investment grade [Member] | Corporates [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value, Less than Twelve Months	20,173
Fair Value, Twelve Months or Longer	222,859
Fair Value, Total	243,032
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(185)
Unrealized Loss, Twelve Months or Longer	(52,139)
Unrealized Loss Total	(52,324)
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	85,338
Fair Value, Total	85,338
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	(20,417)
Unrealized Loss Total	(20,417)
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	20,173
Fair Value, Twelve Months or Longer	81,387
Fair Value, Total	101,560
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	(185)
Unrealized Loss, Twelve Months or Longer	(25,686)
Unrealized Loss Total	(25,871)
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	56,134
Fair Value, Total	56,134
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	(6,036)
Unrealized Loss Total	(6,036)
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	9,697
Fair Value, Total	9,697
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	(10,303)
Unrealized Loss Total	(10,303)
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	21,269
Fair Value, Total	21,269
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	(5,849)
Unrealized Loss Total	(5,849)
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	21,269
Fair Value, Total	21,269
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Unrealized Loss, Less Than Twelve Months	0
Unrealized Loss, Twelve Months or Longer	(5,849)
Unrealized Loss Total	$ (5,849)