Securities in an Unrealized Loss Position (Non-Investment Grade Fixed Maturity Securities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Aggregate fair value of securities in a continuous unrealized loss position	$ 2,971,800	$ 1,536,923
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate fair value of securities in a continuous unrealized loss position	667,212	543,023
Corporate
Schedule of Available-for-sale Securities [Line Items]
Aggregate fair value of securities in a continuous unrealized loss position	1,233,239	442,318
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Aggregate fair value of securities in a continuous unrealized loss position	595,311	288,661
Foreign
Schedule of Available-for-sale Securities [Line Items]
Aggregate fair value of securities in a continuous unrealized loss position	249,199	106,833
Noninvestment Grade Investments at Loss Less than 5 Million
Schedule of Available-for-sale Securities [Line Items]
Number of securities	31
Aggregate fair value of securities in a continuous unrealized loss position	170,788
Gross Unrealized Loss	9,600
Noninvestment Grade Investments at Loss Less than 5 Million | Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of securities	18
Aggregate fair value of securities in a continuous unrealized loss position	106,405
Gross Unrealized Loss	5,246
Noninvestment Grade Investments at Loss Less than 5 Million | Corporate
Schedule of Available-for-sale Securities [Line Items]
Number of securities	9
Aggregate fair value of securities in a continuous unrealized loss position	26,685
Gross Unrealized Loss	4,067
Noninvestment Grade Investments at Loss Less than 5 Million | State and municipal
Schedule of Available-for-sale Securities [Line Items]
Number of securities	2
Aggregate fair value of securities in a continuous unrealized loss position	18,361
Gross Unrealized Loss	47
Noninvestment Grade Investments at Loss Less than 5 Million | Foreign
Schedule of Available-for-sale Securities [Line Items]
Number of securities	2
Aggregate fair value of securities in a continuous unrealized loss position	19,337
Gross Unrealized Loss	$ 240