Related-Party Transactions (Narrative) (Details) $ / shares in Units, $ in Thousands, £ in Millions	1 Months Ended	6 Months Ended
	Dec. 31, 2015 USD ($) $ / shares shares	Jun. 30, 2017 GBP (£) shares	Jun. 30, 2017 USD ($) shares	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2014 USD ($)
Note receivable - affiliated			$ 250,000		$ 250,000
Interest income			$ 2,150	$ 2,150
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%
Reserve Indemnity Agreement, Percent		90.00%	90.00%
Reserve Indemnity Agreement, Amount | £		£ 25
Parent Company [Member]
Note receivable - affiliated						$ 250,000
Debt instrument, stated interest rate			1.72%
Maturity date		Dec. 31, 2023	Dec. 31, 2023