Significant Accounting Policies and Pronouncements (Cumulative-effect Adjustment Upon Adoption) (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other invested assets	[1]	$ 254,050		$ 335,811
Total assets		54,595,457		59,960,850
Total Liabilities		48,167,465		53,679,800
Accumulated other comprehensive income (loss)		(430,694)		(90,124)
Retained earnings		510,750		(776,762)
TOTAL LIABILITIES AND EQUITY		$ 54,595,457		$ 59,960,850
ASU 2016-01 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other invested assets			$ 423
Total assets			423
Income taxes			89
Total Liabilities			89
Accumulated other comprehensive income (loss)			(3)
Retained earnings			337
Total equity			334
TOTAL LIABILITIES AND EQUITY			$ 423

[1] (1)Prior period amounts have been reclassified to conform to current period presentation. See "Adoption of ASU 2016-01" in Note 2 for details.