Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2019	Dec. 31, 2018
Available for sale securities:
Estimated Fair Value		$ 17,632,307	$ 15,908,390
Gross Unrealized Gains		410,779	76,259
Gross Unrealized Losses		(85,689)	(205,900)
Cost or Amortized Cost		17,307,217	16,038,031
OTTI Unrealized Losses		(6)	(75)
Fixed maturities
Available for sale securities:
Estimated Fair Value		16,880,318	14,952,510
Gross Unrealized Gains	[1]	410,552	76,223
Gross Unrealized Losses	[1]	(85,244)	(205,506)
Cost or Amortized Cost	[1]	16,555,010	15,081,793
OTTI Unrealized Losses	[1],[2]	(6)	(75)
Fixed maturities | Corporate bonds
Available for sale securities:
Estimated Fair Value		6,269,993	5,537,548
Gross Unrealized Gains	[1]	198,742	14,476
Gross Unrealized Losses	[1]	(18,420)	(105,428)
Cost or Amortized Cost	[1]	6,089,671	5,628,500
OTTI Unrealized Losses	[1],[2]	0	(69)
Fixed maturities | Mortgage backed securities
Available for sale securities:
Estimated Fair Value		544,033	541,193
Gross Unrealized Gains	[1]	11,185	3,991
Gross Unrealized Losses	[1]	(918)	(3,216)
Cost or Amortized Cost	[1]	533,766	540,418
OTTI Unrealized Losses	[1],[2]	(6)	(6)
Fixed maturities | Municipal bonds
Available for sale securities:
Estimated Fair Value		653,346	1,013,395
Gross Unrealized Gains	[1]	27,095	5,380
Gross Unrealized Losses	[1]	(379)	(11,891)
Cost or Amortized Cost	[1]	626,630	1,019,906
OTTI Unrealized Losses	[1],[2]	0	0
Fixed maturities | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value		754,306	729,442
Gross Unrealized Gains	[1]	27,523	2,650
Gross Unrealized Losses	[1]	(158)	(10,751)
Cost or Amortized Cost	[1]	726,941	737,543
OTTI Unrealized Losses	[1],[2]	0	0
Fixed maturities | US government and government agencies
Available for sale securities:
Estimated Fair Value		5,127,290	3,758,698
Gross Unrealized Gains	[1]	67,957	27,189
Gross Unrealized Losses	[1]	(6,671)	(8,474)
Cost or Amortized Cost	[1]	5,066,004	3,739,983
OTTI Unrealized Losses	[1],[2]	0	0
Fixed maturities | Non-US government securities
Available for sale securities:
Estimated Fair Value		1,873,856	1,771,338
Gross Unrealized Gains	[1]	48,212	14,477
Gross Unrealized Losses	[1]	(52,767)	(50,948)
Cost or Amortized Cost	[1]	1,878,411	1,807,809
OTTI Unrealized Losses	[1],[2]	0	0
Fixed maturities | Asset backed securities
Available for sale securities:
Estimated Fair Value		1,657,494	1,600,896
Gross Unrealized Gains	[1]	29,838	8,060
Gross Unrealized Losses	[1]	(5,931)	(14,798)
Cost or Amortized Cost	[1]	1,633,587	1,607,634
OTTI Unrealized Losses	[1],[2]	0	0
Short-term investments
Available for sale securities:
Estimated Fair Value		751,989	955,880
Gross Unrealized Gains		227	36
Gross Unrealized Losses		(445)	(394)
Cost or Amortized Cost		752,207	956,238
OTTI Unrealized Losses	[2]	$ 0	$ 0

[1] In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.”

[2] Represents the total other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income (“AOCI”). It does not include the change in fair value subsequent to the impairment measurement date. At September 30, 2019 the net unrealized loss related to securities for which a non-credit OTTI was recognized in AOCI was $0.01 million, compared to net unrealized loss of $0.04 million at December 31, 2018