Related-Party Transactions (Narrative) (Details) $ / shares in Units, $ in Thousands, £ in Millions	1 Months Ended	3 Months Ended		9 Months Ended			12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 GBP (£) shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2014 USD ($)
Note receivable - affiliated		$ 250,000			$ 250,000		$ 250,000
Interest income		$ 1,075	$ 1,075		$ 3,225	$ 3,225
Stock repurchase program, number of shares held | shares	9,719,971			9,719,971	9,719,971
Stock repurchase program, value of shares held	$ 1,773,214
Number of preferred shares | shares	1,773.214
Preferred shares par value | $ / shares	$ 1,000
Annual dividend rate	1.75%
Duration of Stop Loss Agreement				12 months	12 months
Reserve Indemnity Agreement, Percent				90.00%	90.00%
Reserve Indemnity Agreement, Amount | £				£ 25
Subject loss reserves							2,336,242
Amount of Business Transfer							1,000,000
Parent Company [Member]
Note receivable - affiliated								$ 250,000
Debt instrument, stated interest rate		1.72%			1.72%
Maturity date				Dec. 31, 2023	Dec. 31, 2023
Bermuda Re [Member]
Amount of Business Transfer							970,000
Adverse development coverage							$ 500,000