Investment Operations Unrealized Losses by Length of Time (Details) $ in Thousands	Jun. 30, 2019 USD ($) issuers securities	Dec. 31, 2018 USD ($) issuers securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | securities	182	709
Available For Sale Securities In Unrealized Loss Positions Qualitative Disclosure Number Of Issuers | issuers	144	465
Corporate Securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 29,173	$ 1,035,176
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(738)	(60,299)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	252,898	207,381
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(21,251)	(30,634)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	282,071	1,242,557
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (21,989)	$ (90,933)
percent of total	83.30%	70.50%
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 3,857	$ 191,365
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(329)	(4,482)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	24,215	74,113
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(446)	(2,760)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	28,072	265,478
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (775)	$ (7,242)
percent of total	2.90%	5.60%
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 89	$ 302,159
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(9,947)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	18,704	148,855
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(526)	(9,443)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	18,793	451,014
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (526)	$ (19,390)
percent of total	2.00%	15.00%
Other asset backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 95,239	$ 250,119
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,401)	(3,397)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	93,275	149,997
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(931)	(1,932)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	188,514	400,116
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (2,332)	$ (5,329)
percent of total	8.80%	4.10%
United States Government and agencies
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2,982	6,474
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(16)	(134)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,982	6,474
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (16)	$ (134)
percent of total	0.10%	0.10%
State, municipal and other government
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 4,219	$ 144,681
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(208)	(3,885)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	6,426	16,943
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(550)	(2,028)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	10,645	161,624
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (758)	$ (5,913)
percent of total	2.90%	4.70%
Debt Securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 132,577	$ 1,923,500
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,676)	(82,010)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	398,500	603,763
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(23,720)	(46,931)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	531,077	2,527,263
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (26,396)	$ (128,941)
percent of total	100.00%	100.00%