Investments - Credit Ratings (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 9,336,295
Fair Value	9,612,888
Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 391,947
Marketable Securities | Portfolio Concentration Risk | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	100.00%
Trading and Available for Sale Securities | Credit Rating Concentration Risk | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	4.10%
AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,609,762
AAA Rated | Trading and Available for Sale Securities | Credit Rating Concentration Risk | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	27.10%
AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,239,096
AA Rated | Trading and Available for Sale Securities | Credit Rating Concentration Risk | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	12.90%
A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 3,297,996
A Rated | Trading and Available for Sale Securities | Credit Rating Concentration Risk | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	34.30%
BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,056,887
BBB Rated | Trading and Available for Sale Securities | Credit Rating Concentration Risk | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	21.40%
Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 17,200
Not Rated | Trading and Available for Sale Securities | Credit Rating Concentration Risk
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	0.20%
U.S. government and agency
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 730,396
Fair Value	736,043
U.S. government and agency | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 0
U.S. government and agency | Marketable Securities | Portfolio Concentration Risk | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	7.70%
U.S. government and agency | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 736,043
U.S. government and agency | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
U.S. government and agency | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
U.S. government and agency | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
U.S. government and agency | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
U.K. government
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	155,261
Fair Value	161,772
U.K. government | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 0
U.K. government | Marketable Securities | Portfolio Concentration Risk | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	1.70%
U.K. government | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 0
U.K. government | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	161,772
U.K. government | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
U.K. government | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
U.K. government | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
Other government
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	684,117
Fair Value	702,857
Other government | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 24,807
Other government | Marketable Securities | Portfolio Concentration Risk | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	7.30%
Other government | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 316,151
Other government | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	154,072
Other government | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	63,270
Other government | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	144,557
Other government | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
Corporate
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	5,479,839
Fair Value	5,697,067
Corporate | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 311,167
Corporate | Marketable Securities | Portfolio Concentration Risk | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	59.20%
Corporate | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 149,108
Corporate | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	619,707
Corporate | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	2,911,867
Corporate | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	1,703,328
Corporate | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	1,890
Municipal
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	157,868
Fair Value	167,882
Municipal | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 0
Municipal | Marketable Securities | Portfolio Concentration Risk | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	1.70%
Municipal | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 16,381
Municipal | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	77,291
Municipal | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	50,938
Municipal | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	23,272
Municipal | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
Residential mortgage-backed
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	466,932
Fair Value	471,836
Residential mortgage-backed | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 34,055
Residential mortgage-backed | Marketable Securities | Portfolio Concentration Risk | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	4.90%
Residential mortgage-backed | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 381,502
Residential mortgage-backed | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	47,489
Residential mortgage-backed | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	2,295
Residential mortgage-backed | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	1,882
Residential mortgage-backed | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	4,613
Commercial mortgage-backed
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	882,603
Fair Value	900,029
Commercial mortgage-backed | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,224
Commercial mortgage-backed | Marketable Securities | Portfolio Concentration Risk | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	9.40%
Commercial mortgage-backed | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 632,461
Commercial mortgage-backed | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	89,347
Commercial mortgage-backed | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	95,508
Commercial mortgage-backed | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	66,573
Commercial mortgage-backed | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	9,916
Asset-backed
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	779,279
Fair Value	775,402
Asset-backed | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 15,694
Asset-backed | Marketable Securities | Portfolio Concentration Risk | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Percent of total investments	8.10%
Asset-backed | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 378,116
Asset-backed | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	89,418
Asset-backed | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	174,118
Asset-backed | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	117,275
Asset-backed | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 781